Comparing .equals and == Operators for Null Checks in JAVA

Value Comparison in JAVA

Value Comparison in JAVA
When checking for null or comparing values, the == operator and .equals() method are commonly used. This article explains when to use the == operator and .equals() method in JAVA and how they are structured.

① ““.equals()

In "A".equals(B), A is the value you want to compare, and B is the object.

"value_to_compare".equals(object)

Looking at ”“.equals(String), it uses the equals method of the String class.

The equals Method of the String Class

public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        // The instanceOf operator checks if the object is an instance of a class or a subclass.
        // Here, it checks if it is a String.
        if (anObject instanceof String) {		
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }

② Differences Between == and .equals()

The == operator compares values for primitive types like int and boolean, while for reference types, it compares memory addresses.

.equals: Call by Value (compares values)
== null: Call by Reference (compares addresses)

In primitive types, it can also be seen as comparing addresses because it references specific constants in the Constant Pool.
When referencing the same constant, the address is the same, so the values are considered equal.

Comparing Values and Addresses (“.equals() vs == null”)

String test1 = null;
System.out.println(".equals(test1) :: " + "".equals(test1)); // Value comparison: false
System.out.println("null==test1 :: " + null==test1); // Address comparison: true

String test2 = "";
System.out.println(".equals(test2) :: " + "".equals(test2)); // Value comparison: true
System.out.println("null==test2 :: " + null==test2); // Address comparison: false

Checking for Null

String testString = "";
if(testString == null || "".equals(testString) ){
    System.out.println("The value is null.");
}