Java Strings

Strings are used for storing text.

A String variable contains a collection of characters surrounded by double quotes:

Example

Create a variable of type String and assign it a value:

String greeting = "Hello";

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String Length

A String in Java is actually an object, which contain methods that can perform certain operations on strings. For example, the length of a string can be found with the length() method:

Example

String txt = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

System.out.println("The length of the txt string is: " + txt.length());

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More String Methods

There are many string methods available, for example toUpperCase() and toLowerCase():

Example

String txt = "Hello World";

System.out.println(txt.toUpperCase()); // Outputs "HELLO WORLD"

System.out.println(txt.toLowerCase()); // Outputs "hello world"

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Finding a Character in a String

The indexOf() method returns the index (the position) of the first occurrence of a specified text in a string (including whitespace):

Example

String txt = "Please locate where 'locate' occurs!";

System.out.println(txt.indexOf("locate")); // Outputs 7

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Java counts positions from zero.

0 is the first position in a string, 1 is the second, 2 is the third ...

String Concatenation

The + operator can be used between strings to combine them. This is called concatenation:

Example

String firstName = "John";

String lastName = "Doe";

System.out.println(firstName + " " + lastName);

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Note that we have added an empty text (" ") to create a space between firstName and lastName on print.

You can also use the concat() method to concatenate two strings:

Example

String firstName = "John ";

String lastName = "Doe";

System.out.println(firstName.concat(lastName));

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Special Characters

Because strings must be written within quotes, Java will misunderstand this string, and generate an error:

String txt = "We are the so-called "Vikings" from the north.";

The solution to avoid this problem, is to use the backslash escape character.

The backslash (\) escape character turns special characters into string characters:

Escape character

Result

Description

\'

'

Single quote

\"

"

Double quote

\\

\

Backslash

The sequence \" inserts a double quote in a string:

Example

String txt = "We are the so-called \"Vikings\" from the north.";

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The sequence \' inserts a single quote in a string:

Example

String txt = "It\'s alright.";

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The sequence \\ inserts a single backslash in a string:

Example

String txt = "The character \\ is called backslash.";

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Six other escape sequences are valid in Java:

Code

Result

Try it

\n

New Line

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\r

Carriage Return

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\t

Tab

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\b

Backspace

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\f

Form Feed

Adding Numbers and Strings

WARNING!

Java uses the + operator for both addition and concatenation.

Numbers are added. Strings are concatenated.

If you add two numbers, the result will be a number:

Example

int x = 10;

int y = 20;

int z = x + y; // z will be 30 (an integer/number)

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If you add two strings, the result will be a string concatenation:

Example

String x = "10";

String y = "20";

String z = x + y; // z will be 1020 (a String)

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If you add a number and a string, the result will be a string concatenation:

Example

String x = "10";

int y = 20;

String z = x + y; // z will be 1020 (a String)

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Complete String Reference

For a complete reference of String methods, go to our Java String Methods Reference.

The reference contains descriptions and examples of all string methods.

Java Math

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The Java Math class has many methods that allows you to perform mathematical tasks on numbers.

Math.max(x,y)

The Math.max(x,y) method can be used to find the highest value of x and y:

Example

Math.max(5, 10);

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Math.min(x,y)

The Math.min(x,y) method can be used to find the lowest value of x and y:

Example

Math.min(5, 10);

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Math.sqrt(x)

The Math.sqrt(x) method returns the square root of x:

Example

Math.sqrt(64);

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Math.abs(x)

The Math.abs(x) method returns the absolute (positive) value of x:

Example

Math.abs(-4.7);

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Random Numbers

Math.random() returns a random number between 0.0 (inclusive), and 1.0 (exclusive):

Example

Math.random();

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To get more control over the random number, e.g. you only want a random number between 0 and 100, you can use the following formula:

Example

int randomNum = (int)(Math.random() * 101); // 0 to 100

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Complete Math Reference

For a complete reference of Math methods, go to our Java Math Methods Reference.

Java Booleans

Very often, in programming, you will need a data type that can only have one of two values, like:

• YES / NO

• ON / OFF

• TRUE / FALSE

For this, Java has a boolean data type, which can take the values true or false.

Boolean Values

A boolean type is declared with the boolean keyword and can only take the values true or false:

Example

boolean isJavaFun = true;

boolean isFishTasty = false;

System.out.println(isJavaFun); // Outputs true

System.out.println(isFishTasty); // Outputs false

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However, it is more common to return boolean values from boolean expressions, for conditional testing (see below).

Boolean Expression

A Boolean expression is a Java expression that returns a Boolean value: true or false.

You can use a comparison operator, such as the greater than (>) operator to find out if an expression (or a variable) is true:

Example

int x = 10;

int y = 9;

System.out.println(x > y); // returns true, because 10 is higher than 9

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Or even easier:

Example

System.out.println(10 > 9); // returns true, because 10 is higher than 9

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In the examples below, we use the equal to (==) operator to evaluate an expression:

Example

int x = 10;

System.out.println(x == 10); // returns true, because the value of x is equal to 10

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Example

System.out.println(10 == 15); // returns false, because 10 is not equal to 15

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The Boolean value of an expression is the basis for all Java comparisons and conditions.

You will learn more about conditions in the next chapter.