Equals, equalsIgnoreCase

Java strings contain characters. These can be letters (lowercase and uppercase), digits, spaces. We want to compare 2 strings for equality.

With equals, we test the chars in 2 string objects. If all the characters are the same, equals() returns true. With equalsIgnoreCase, lowercase and uppercase are considered the same.

First example

Let us use equals() in a simple program. We introduce three test strings. They all contain the same characters, but the third one is not a string literal.

public class Program {

    public static void main(String[] args) {

        String test = "1234";
        String test2 = "1234";
        String test3 = Integer.toString(1234);

        if (test.equals(test2)) {
            System.out.println(true);
        }

        if (test.equals(test3)) {
            System.out.println(true);
        }

        if (!test.equals("456")) {
            System.out.println(false);
        }
    }
}
true
true
false

Equals operator

The == and != operators can be used on string references. But this does not handle the internal characters in the String objects. It just tests the references.

public class Program {

    public static void main(String[] args) {

        String test = "carrot1";
        String test2 = "carrot" + Integer.toString(1);
        // Test the equals, not equals operator.
        if (test != test2) {
            System.out.println("Not equal");
            System.out.println(test);
            System.out.println(test2);
        }
        // The equals method is different.
        if (test.equals(test2)) {
            System.out.println("Equals = true");
            System.out.println(test);
            System.out.println(test2);
        }
    }
}
Not equal
carrot1
carrot1
Equals = true
carrot1
carrot1

EqualsIgnoreCase

Sometimes we do not care if a letter is uppercase or lower. The EqualsIgnoreCase method treats both cases as equal. It does not care about upper and lower.

public class Program {

    public static void main(String[] args) {

        String name = "APPLE";
        String name2 = name.toLowerCase();

        // The uppercase and lowercase letters are considered equal.
        if (name.equalsIgnoreCase(name2)) {
            System.out.println(true);
            System.out.println(name);
            System.out.println(name2);
        }
    }
}
true
APPLE
apple

Null

This program shows that a non-null string never equals a null string. Equals() will always return false when passed a null variable. But it throws no exception.

public class Program {

    public static void main(String[] args) {

        String value = "radish";
        String other = null;

        // The null literal is not equal to any String.
        if (!value.equals(other)) {
            System.out.println(true);
        }
    }
}
true

ContentEquals

This method tests the value of objects through their CharSequences. So it can compare a String against a StringBuilder. It is called on a String instance.

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

        // Create 2 strings with the same character data.
        String part1 = "cat100";

        String part2 = "cat";
        part2 += String.valueOf(100);

        // Create a StringBuilder with the same character data.
        StringBuilder builder = new StringBuilder();
        builder.append("cat");
        builder.append(100);

        // Use contentEquals to compare character data.
        if (part1.contentEquals(part2)) {
            System.out.println("True 1");
        }

        if (part1.contentEquals(builder)) {
            System.out.println("True 2");
        }
    }
}
True 1
True 2

With Strings, we must keep in mind the difference between references and objects. A string variable is a reference, but this reference points to an object.

With equality operators (like "==") we can test two references for equality. But to check the data within String objects, we must use equals or equalsIgnoreCase.