How to compare strings in Java

Gareth D.

The Problem

How do I compare strings in Java?

The Solution

To compare strings in Java for equality, you should use String.equals().

public class Main {

    public static void main(String[] arg) {
        String str1 = "java";
        String str2 = "java";
        System.out.println(str1.equals(str2));
    }
}

Output:

true

If uppercase and lowercase difference isn’t important, you can use String.equalsIgnoreCase().

public class Main {

    public static void main(String[] arg) {
        String str1 = "java";
        String str2 = "Java";
        System.out.println(str1.equalsIgnoreCase(str2));
    }
}

Output:

true

If you want to find out if a String is “bigger” or “smaller” than another string (that is, whether it comes before or after alphabetically), use String.compareTo().

public class Main {

    public static void main(String[] arg) {
        String str1 = "aaa";
        String str2 = "bbb";
        String str3 = "ccc";
        String str4 = "bbb";
        System.out.println(str2.compareTo(str3));
        System.out.println(str2.compareTo(str1));
        System.out.println(str2.compareTo(str4));
    }
}

Output:

-1
1
0

The first output is -1, indicating that str2 ("bbb") comes before str3 ("ccc").

The second output is 1, indicating that str2 ("bbb") comes after str1 ("aaa").

The third output is 0, indicating that str2 ("bbb") is equal to str4 ("bbb").

Why you shouldn’t use == for string comparison

Strings in Java are stored on the heap, and the variable only stores a reference to where the real string value can be found.

If you use == to compare strings, you might get unexpected results.

public class Main {

    public static void main(String[] arg) {
        String str1 = new String("java");
        String str2 = new String("java");
        System.out.println(str1 == str2);
    }
}

Output:

false

Here, the two strings have equal values but these values are not stored in the variables str1 and str2.

When dealing with primitive types like int, any variables with the same value will also have the same identity code. But when dealing with non-primitive types like String, this is not the case.

public class Main {

    public static void main(String[] arg) {
        String str1 = new String("java");
        String str2 = new String("java");
        int int1 = 1;
        int int2 = 1;

        System.out.println(System.identityHashCode(int1));
        System.out.println(System.identityHashCode(int2));

        System.out.println(System.identityHashCode(str1));
        System.out.println(System.identityHashCode(str2));
    }
}

Output:

808993268
808993268
1229416514
2016447921

What makes this confusing is that often == will return true if you compare two strings with the same value. In the code above we used new String("java") to create two separate strings, but if we create the strings without the explicit new keyword, Java will do some optimization in the background and only store the string once.

public class Main {

    public static void main(String[] arg) {
        String str1 = "java";
        String str2 = "java";

        System.out.println(System.identityHashCode(str1));
        System.out.println(System.identityHashCode(str2));

        // do not do this
        System.out.println(str1 == str2);
    }
}

Here, the two strings have the same hashcode, and == will tell you that they are the same string. This is a side effect of Java’s memory optimization and shouldn’t be relied on, as it may not always behave consistently.