How compare() method works in Java

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Prerequisite: Comparator Interface in Java, TreeSet in Java
The compare() method in Java compares two class specific objects (x, y) given as parameters. It returns the value:

0: if (x==y)
-1: if (x < y)
1: if (x > y)

Syntax:

public int compare(Object obj1, Object obj2)

where obj1 and obj2 are the two objects to be compared using compare() method.

Example:

To show working of compare() method using Integer Class.

Java

// Java program to demonstrate working
// of compare() method using Integer Class
 
import java.lang.Integer;
 
class Gfg {
 
    // driver code
    public static void main(String args[])
    {
        int a = 10;
        int b = 20;
 
        // as 10 less than 20,
        // Output will be a value less than zero
        System.out.println(Integer.compare(a, b));
 
        int x = 30;
        int y = 30;
 
        // as 30 equals 30,
        // Output will be zero
        System.out.println(Integer.compare(x, y));
 
        int w = 15;
        int z = 8;
 
        // as 15 is greater than 8,
        // Output will be a value greater than zero
        System.out.println(Integer.compare(w, z));
    }
}

Output:

-1
0
1

How is the return value evaluated:

The internal working of the compare() method can be visualized with the help of below pseudocode:

Java

// Converting the two objects to integer
// for comparison
int intObj1 = (int)obj1;
int intObj2 = (int)obj2;
 
// Get the difference
int difference = intObj1 - intObj2;
 
if (difference == 0) {
 
    // Both are equal
    return 0;
}
else if (difference < 0) {
 
    // obj1 < obj2
    return -1;
}
else {
 
    // obj1 > obj2
    return 1;
}

Visualizing the compare() method with this approach:

Java

// Java program to demonstrate working
// of compare() method
 
import java.lang.Integer;
 
class Gfg {
 
    // Function to compare both objects
    public static int compare(Object obj1, Object obj2)
    {
 
        // Converting the two objects to integer
        // for comparison
        int intObj1 = (int)obj1;
        int intObj2 = (int)obj2;
 
        // Get the difference
        int difference = intObj1 - intObj2;
 
        if (difference == 0) {
 
            // Both are equal
            return 0;
        }
        else if (difference < 0) {
 
            // obj1 < obj2
            return -1;
        }
        else {
 
            // obj1 > obj2
            return 1;
        }
    }
 
    // driver code
    public static void main(String args[])
    {
        int a = 10;
        int b = 20;
 
        // as 10 less than 20,
        // Output will be a value less than zero
        System.out.println(compare(a, b));
 
        int x = 30;
        int y = 30;
 
        // as 30 equals 30,
        // Output will be zero
        System.out.println(compare(x, y));
 
        int w = 15;
        int z = 8;
 
        // as 15 is greater than 8,
        // Output will be a value greater than zero
        System.out.println(compare(w, z));
    }
}

Output:

-1
0
1

Various possible implementations of compare() method

public int compare(Object obj1, Object obj2)
{
    Integer I1 = (Integer)obj1; // typecasting object type into integer type
    Integer I2 = (Integer)obj2; // same as above ..
    // 1.
    return I1.compareTo(I2); // ascending order [0, 5, 10, 15, 20]
    // 2.
    return -I1.compareTo(I2); // descending order [20, 15, 10, 5, 0]
    // 3.
    return I2.compareTo(I1); // descending order [20, 15, 10, 5, 0]
    // 4.
    return -I2.compareTo(I1); // ascending order [0, 5, 10, 15, 20]
    // 5.
    return +1; // insertion order [10, 0, 15, 5, 20, 20]
    // 6.
    return -1; // reverse of insertion order [20, 20, 5, 15, 0, 10]
    // 7.
    return 0; // only first element [10]
}