Converting Between Numbers and Strings Converting Strings to Numbers

Converting Strings to Numbers
Frequently, a program ends up with numeric data in a string object—a value entered by the user, for example.

The Number subclasses that wrap primitive numeric types ( Byte, Integer, Double, Float, Long, and Short) each provide a class method named valueOf that converts a string to an object of that type. Here is an example, ValueOfDemo , that gets two strings from the command line, converts them to numbers, and performs arithmetic operations on the values:

public class ValueOfDemo {
public static void main(String[] args) {

//this program requires two arguments on the command line
if (args.length == 2) {
//convert strings to numbers
float a = (Float.valueOf(args[0]) ).floatValue();
float b = (Float.valueOf(args[1]) ).floatValue();

//do some arithmetic
System.out.println("a + b = " + (a + b) );
System.out.println("a - b = " + (a - b) );
System.out.println("a * b = " + (a * b) );
System.out.println("a / b = " + (a / b) );
System.out.println("a % b = " + (a % b) );
} else {
System.out.println("This program requires two command-line arguments.");
}
}
}

The following is the output from the program when you use 4.5 and 87.2 for the command-line arguments:

a + b = 91.7
a - b = -82.7
a * b = 392.4
a / b = 0.0516055
a % b = 4.5

Note: Each of the Number subclasses that wrap primitive numeric types also provides a parseXXXX() method (for example, parseFloat()) that can be used to convert strings to primitive numbers. Since a primitive type is returned instead of an object, the parseFloat() method is more direct than the valueOf() method. For example, in the ValueOfDemo program, we could use:

float a = Float.parseFloat(args[0]);
float b = Float.parseFloat(args[1]);

Converting Numbers to Strings
Sometimes you need to convert a number to a string because you need to operate on the value in its string form. There are several easy ways to convert a number to a string:

int i;
String s1 = "" + i; //Concatenate "i" with an empty string;
//conversion is handled for you.

or

String s2 = String.valueOf(i); //The valueOf class method.

Each of the Number subclasses includes a class method, toString(), that will convert its primitive type to a string. For example:

int i;
double d;
String s3 = Integer.toString(i);
String s4 = Double.toString(d);

The ToStringDemo example uses the toString method to convert a number to a string. The program then uses some string methods to compute the number of digits before and after the decimal point:

public class ToStringDemo {

public static void main(String[] args) {
double d = 858.48;
String s = Double.toString(d);

int dot = s.indexOf('.');

System.out.println(dot + " digits before decimal point.");
System.out.println( (s.length() - dot - 1) +
" digits after decimal point.");
}
}

The output of this program is:

3 digits before decimal point.
2 digits after decimal point.

Strings

Strings, which are widely used in Java programming, are a sequence of characters. In the Java programming language, strings are objects.

The Java platform provides the String class to create and manipulate strings.
Creating Strings
The most direct way to create a string is to write:

String greeting = "Hello world!";

In this case, "Hello world!" is a string literal—a series of characters in your code that is enclosed in double quotes. Whenever it encounters a string literal in your code, the compiler creates a String object with its value—in this case, Hello world!.

As with any other object, you can create String objects by using the new keyword and a constructor. The String class has 11 constructors that allow you to provide the initial value of the string using different sources, such as an array of characters:

char[] helloArray = { 'h', 'e', 'l', 'l', 'o', '.'};
String helloString = new String(helloArray);
System.out.println(helloString);

The last line of this code snippet displays hello.

Note: The String class is immutable, so that once it is created a String object cannot be changed. The String class has a number of methods, some of which will be discussed below, that appear to modify strings. Since strings are immutable, what these methods really do is create and return a new string that contains the result of the operation.

String Length
Methods used to obtain information about an object are known as accessor methods. One accessor method that you can use with strings is the length() method, which returns the number of characters contained in the string object. After the following two lines of code have been executed, len equals 17:

String palindrome = "Dot saw I was Tod";
int len = palindrome.length();

A palindrome is a word or sentence that is symmetric—it is spelled the same forward and backward, ignoring case and punctuation. Here is a short and inefficient program to reverse a palindrome string. It invokes the String method charAt(i), which returns the ith character in the string, counting from 0.

public class StringDemo {
public static void main(String[] args) {
String palindrome = "Dot saw I was Tod";
int len = palindrome.length();
char[] tempCharArray = new char[len];
char[] charArray = new char[len];

// put original string in an array of chars
for (int i = 0; i < len; i++) {
tempCharArray[i] = palindrome.charAt(i);
}

// reverse array of chars
for (int j = 0; j < len; j++) {
charArray[j] = tempCharArray[len - 1 - j];
}

String reversePalindrome = new String(charArray);
System.out.println(reversePalindrome);
}
}

Running the program produces this output:

doT saw I was toD

To accomplish the string reversal, the program had to convert the string to an array of characters (first for loop), reverse the array into a second array (second for loop), and then convert back to a string. The String class includes a method, getChars(), to convert a string, or a portion of a string, into an array of characters so we could replace the first for loop in the program above with

palindrome.getChars(0, len, tempCharArray, 0);

Concatenating Strings
The String class includes a method for concatenating two strings:

string1.concat(string2);

This returns a new string that is string1 with string2 added to it at the end.

You can also use the concat() method with string literals, as in:

"My name is ".concat("Rumplestiltskin");

Strings are more commonly concatenated with the + operator, as in

"Hello," + " world" + "!"

which results in

"Hello, world!"

The + operator is widely used in print statements. For example:

String string1 = "saw I was ";
System.out.println("Dot " + string1 + "Tod");

which prints

Dot saw I was Tod

Such a concatenation can be a mixture of any objects. For each object that is not a String, its toString() method is called to convert it to a String.

Note: The Java programming language does not permit literal strings to span lines in source files, so you must use the + concatenation operator at the end of each line in a multi-line string. For example,

String quote = "Now is the time for all good " +
"men to come to the aid of their country.";

Breaking strings between lines using the + concatenation operator is, once again, very common in print statements.

Creating Format Strings
You have seen the use of the printf() and format() methods to print output with formatted numbers. The String class has an equivalent class method, format(), that returns a String object rather than a PrintStream object.

Using String's static format() method allows you to create a formatted string that you can reuse, as opposed to a one-time print statement. For example, instead of

System.out.printf("The value of the float variable is %f, while the value of the " +
"integer variable is %d, and the string is %s", floatVar, intVar, stringVar);

you can write

String fs;
fs = String.format("The value of the float variable is %f, while the value of the " +
"integer variable is %d, and the string is %s", floatVar, intVar, stringVar);
System.out.println(fs);

Escape Sequences

A character preceded by a backslash (\) is an escape sequence and has special meaning to the compiler. The newline character (\n) has been used frequently in this tutorial in System.out.println() statements to advance to the next line after the string is printed. The following table shows the Java escape sequences:

Escape Sequences Escape Sequence Description
\t Insert a tab in the text at this point.
\b Insert a backspace in the text at this point.
\n Insert a newline in the text at this point.
\r Insert a carriage return in the text at this point.
\f Insert a formfeed in the text at this point.
\' Insert a single quote character in the text at this point.
\" Insert a double quote character in the text at this point.
\\ Insert a backslash character in the text at this point.

When an escape sequence is encountered in a print statement, the compiler interprets it accordingly. For example, if you want to put quotes within quotes you must use the escape sequence, \", on the interior quotes. To print the sentence

She said "Hello!" to me.

you would write

System.out.println("She said \"Hello!\" to me.");

Characters

Most of the time, if you are using a single character value, you will use the primitive char type. For example:

char ch = 'a';
char uniChar = '\u039A'; // Unicode for uppercase Greek omega character
char[] charArray ={ 'a', 'b', 'c', 'd', 'e' }; // an array of chars

There are times, however, when you need to use a char as an object—for example, as a method argument where an object is expected. The Java programming language provides a wrapper class that "wraps" the char in a Character object for this purpose. An object of type Character contains a single field, whose type is char. This Character class also offers a number of useful class (i.e., static) methods for manipulating characters.

You can create a Character object with the Character constructor:

Character ch = new Character('a');

The Java compiler will also create a Character object for you under some circumstances. For example, if you pass a primitive char into a method that expects an object, the compiler automatically converts the char to a Character for you. This feature is called autoboxing—or unboxing, if the conversion goes the other way.

Here is an example of boxing,

Character ch = 'a'; // the primitive char 'a' is boxed into the Character object ch

and here is an example of both boxing and unboxing,

Character test(Character c) {...} // method parameter and return type = Character object

char c = test('x'); // primitive 'x' is boxed for method test, return is unboxed to char 'c'

Note: The Character class is immutable, so that once it is created, a Character object cannot be changed.

The following table lists some of the most useful methods in the Character class, but is not exhaustive. For a complete listing of all methods in this class (there are more than 50), refer to the java.lang.Character API specification.

Useful Methods in the Character Class Method Description
boolean isLetter(char ch)
boolean isDigit(char ch) Determines whether the specified char value is a letter or a digit, respectively.
boolean isWhiteSpace(char ch) Determines whether the specified char value is white space.
boolean isUpperCase(char ch)
boolean isLowerCase(char ch) Determines whether the specified char value is uppercase or lowercase, respectively.
char toUpperCase(char ch)
char toLowerCase(char ch) Returns the uppercase or lowercase form of the specified char value.
toString(char ch) Returns a String object representing the specified character value—that is, a one-character string.

Summary of Numbers and Questions and Exercises: Numbers

You use one of the wrapper classes – Byte, Double, Float, Integer, Long, or Short – to wrap a number of primitive type in an object. The Java compiler automatically wraps (boxes) primitives for you when necessary and unboxes them, again when necessary.

The Number classes include constants and useful class methods. The MIN_VALUE and MAX_VALUE constants contain the smallest and largest values that can be contained by an object of that type. The byteValue, shortValue, and similar methods convert one numeric type to another. The valueOf method converts a string to a number, and the toString method converts a number to a string.

To format a string containing numbers for output, you can use the printf() or format() methods in the PrintStream class. Alternatively, you can use the NumberFormat class to customize numerical formats using patterns.

The Math class contains a variety of class methods for performing mathematical functions, including exponential, logarithmic, and trigonometric methods. Math also includes basic arithmetic functions, such as absolute value and rounding, and a method, random(), for generating random numbers.
Questions

1. Use the API documentation to find the answers to the following questions:

a. What Integer method can you use to convert an int into a string that expresses the number in hexadecimal? For example, what method converts the integer 65 into the string "41"?

b. What Integer method would you use to convert a string expressed in base 5 into the equivalent int? For example, how would you convert the string "230" into the integer value 65? Show the code you would use to accomplish this task.

c. What Double method can you use to detect whether a floating-point number has the special value Not a Number (NaN)?

2. What is the value of the following expression, and why?

Integer.valueOf(1).equals(Long.valueOf(1))

Exercises

1. Change MaxVariablesDemo to show minimum values instead of maximum values. You can delete all code related to the variables aChar and aBoolean. What is the output?

2. Create a program that reads an unspecified number of integer arguments from the command line and adds them together. For example, suppose that you enter the following:

java Adder 1 3 2 10

The program should display 16 and then exit. The program should display an error message if the user enters only one argument. You can base your program on ValueOfDemo.

3. Create a program that is similar to the previous one but has the following differences:

* Instead of reading integer arguments, it reads floating-point arguments.
* It displays the sum of the arguments, using exactly two digits to the right of the decimal point.

For example, suppose that you enter the following:

java FPAdder 1 1e2 3.0 4.754

The program would display 108.75. Depending on your locale, the decimal point might be a comma (,) instead of a period (.).

Exponential and Logarithmic Methods and Trigonometric Methods and Random Numbers

The next table lists exponential and logarithmic methods of the Math class.

Exponential and Logarithmic Methods Method Description
double exp(double d) Returns the base of the natural logarithms, e, to the power of the argument.
double log(double d) Returns the natural logarithm of the argument.
double pow(double base, double exponent) Returns the value of the first argument raised to the power of the second argument.
double sqrt(double d) Returns the square root of the argument.

The following program, ExponentialDemo , displays the value of e, then calls each of the methods listed in the previous table on arbitrarily chosen numbers:

public class ExponentialDemo {
public static void main(String[] args) {
double x = 11.635;
double y = 2.76;

System.out.printf("The value of e is %.4f%n", Math.E);
System.out.printf("exp(%.3f) is %.3f%n", x, Math.exp(x));
System.out.printf("log(%.3f) is %.3f%n", x, Math.log(x));
System.out.printf("pow(%.3f, %.3f) is %.3f%n", x, y, Math.pow(x, y));
System.out.printf("sqrt(%.3f) is %.3f%n", x, Math.sqrt(x));
}
}

Here's the output you'll see when you run ExponentialDemo:

The value of e is 2.7183
exp(11.635) is 112983.831
log(11.635) is 2.454
pow(11.635, 2.760) is 874.008
sqrt(11.635) is 3.411

Trigonometric Methods
The Math class also provides a collection of trigonometric functions, which are summarized in the following table. The value passed into each of these methods is an angle expressed in radians. You can use the toRadians method to convert from degrees to radians.

Trigonometric Methods Method Description
double sin(double d) Returns the sine of the specified double value.
double cos(double d) Returns the cosine of the specified double value.
double tan(double d) Returns the tangent of the specified double value.
double asin(double d) Returns the arcsine of the specified double value.
double acos(double d) Returns the arccosine of the specified double value.
double atan(double d) Returns the arctangent of the specified double value.
double atan2(double y, double x) Converts rectangular coordinates (x, y) to polar coordinate (r, theta) and returns theta.
double toDegrees(double d)
double toRadians(double d) Converts the argument to degrees or radians.

Here's a program, TrigonometricDemo , that uses each of these methods to compute various trigonometric values for a 45-degree angle:

public class TrigonometricDemo {
public static void main(String[] args) {
double degrees = 45.0;
double radians = Math.toRadians(degrees);

System.out.format("The value of pi is %.4f%n", Math.PI);
System.out.format("The sine of %.1f degrees is %.4f%n", degrees,
Math.sin(radians));
System.out.format("The cosine of %.1f degrees is %.4f%n", degrees,
Math.cos(radians));
System.out.format("The tangent of %.1f degrees is %.4f%n", degrees,
Math.tan(radians));
System.out.format("The arcsine of %.4f is %.4f degrees %n",
Math.sin(radians),
Math.toDegrees(Math.asin(Math.sin(radians))));
System.out.format("The arccosine of %.4f is %.4f degrees %n",
Math.cos(radians),
Math.toDegrees(Math.acos(Math.cos(radians))));
System.out.format("The arctangent of %.4f is %.4f degrees %n",
Math.tan(radians),
Math.toDegrees(Math.atan(Math.tan(radians))));

}
}



The output of this program is as follows:

The value of pi is 3.1416
The sine of 45.0 degrees is 0.7071
The cosine of 45.0 degrees is 0.7071
The tangent of 45.0 degrees is 1.0000
The arcsine of 0.7071 is 45.0000 degrees
The arccosine of 0.7071 is 45.0000 degrees
The arctangent of 1.0000 is 45.0000 degrees

Random Numbers
The random() method returns a pseudo-randomly selected number between 0.0 and 1.0. The range includes 0.0 but not 1.0. In other words: 0.0 <= Math.random() < 1.0. To get a number in a different range, you can perform arithmetic on the value returned by the random method. For example, to generate an integer between 0 and 9, you would write:

int number = (int)(Math.random() * 10);

By multiplying the value by 10, the range of possible values becomes 0.0 <= number < 10.0.

Using Math.random works well when you need to generate a single random number. If you need to generate a series of random numbers, you should create an instance of java.util.Random and invoke methods on that object to generate numbers.

Beyond Basic Arithmetic(Constants and Basic Methods)

The Java programming language supports basic arithmetic with its arithmetic operators: +, -, *, /, and %. The Math class in the java.lang package provides methods and constants for doing more advanced mathematical computation.

The methods in the Math class are all static, so you call them directly from the class, like this:

Math.cos(angle);

Note : Using the static import language feature, you don't have to write Math in front of every math function:

import static java.lang.Math.*;

This allows you to invoke the Math class methods by their simple names. For example:

cos(angle);

Constants and Basic Methods
The Math class includes two constants:

* Math.E, which is the base of natural logarithms, and
* Math.PI, which is the ratio of the circumference of a circle to its diameter.

The Math class also includes more than 40 static methods. The following table lists a number of the basic methods.

Basic Math Methods Method Description
double abs(double d)
float abs(float f)
int abs(int i)
long abs(long lng) Returns the absolute value of the argument.
double ceil(double d) Returns the smallest integer that is greater than or equal to the argument. Returned as a double.
double floor(double d) Returns the largest integer that is less than or equal to the argument. Returned as a double.
double rint(double d) Returns the integer that is closest in value to the argument. Returned as a double.
long round(double d)
int round(float f) Returns the closest long or int, as indicated by the method's return type, to the argument.
double min(double arg1, double arg2)
float min(float arg1, float arg2)
int min(int arg1, int arg2)
long min(long arg1, long arg2) Returns the smaller of the two arguments.
double max(double arg1, double arg2)
float max(float arg1, float arg2)
int max(int arg1, int arg2)
long max(long arg1, long arg2) Returns the larger of the two arguments.

The following program, BasicMathDemo , illustrates how to use some of these methods:

public class BasicMathDemo {
public static void main(String[] args) {
double a = -191.635;
double b = 43.74;
int c = 16, d = 45;

System.out.printf("The absolute value of %.3f is %.3f%n", a, Math.abs(a));
System.out.printf("The ceiling of %.2f is %.0f%n", b, Math.ceil(b));
System.out.printf("The floor of %.2f is %.0f%n", b, Math.floor(b));
System.out.printf("The rint of %.2f is %.0f%n", b, Math.rint(b));
System.out.printf("The max of %d and %d is %d%n",c, d, Math.max(c, d));
System.out.printf("The min of of %d and %d is %d%n",c, d, Math.min(c, d));


}
}

Here's the output from this program:

The absolute value of -191.635 is 191.635
The ceiling of 43.74 is 44
The floor of 43.74 is 43
The rint of 43.74 is 44
The max of 16 and 45 is 45
The min of 16 and 45 is 16