Java equals() Method vs == Operator – Key Differences

equalsjava

I know that equals() will compare the value of objects, the '==' operator will check if the variable point to the same memory.

I do not understand how equals() compare the value of objects, for example:

class Test {
    public Test(int x, float y) {
        this.x = x;
        this.y = y;
    }

    int x,
    float y;
}

Test test1 = new Test(1,2.0);
Test test2 = new Test(1,2.0);

So if I use equals(), will it compare each properties in each object?

And what about if we are talking about String? using equals() and operator “==”, do we still need to override the equals()?

Best Answer

No, if you don't override the equals-method in your class, then equals is the same as ==. See the documentation for this:

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

The documentation also states what requirements there are for equals methods in case you want to implement it.

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Java – Why Are These == but Not `equals()`?

(small) Integer instances are cached, so the invariant x == y is holded for small instances (actually -127 +128, depends on JVM):

Integer a = 10;
Integer b = 10;

assert(a == b); // ok, same instance reused

a = 1024;
b = 1024;

assert(a == b); // fail, not the same instance....
assert(a.equals(b)); // but same _value_

EDIT

4) and 5) yield false because equals check types: X is an Integer whereas Y is a Short. This is the java.lang.Integer#equals method:

public boolean equals(Object obj) {
    if (obj instanceof Integer) {
        return value == ((Integer)obj).intValue();
    }

    return false;
}

Java == vs equals() – Difference Between == and equals() in Java

In general, the answer to your question is "yes", but...

.equals(...) will only compare what it is written to compare, no more, no less.
If a class does not override the equals method, then it defaults to the equals(Object o) method of the closest parent class that has overridden this method.
If no parent classes have provided an override, then it defaults to the method from the ultimate parent class, Object, and so you're left with the Object#equals(Object o) method. Per the Object API this is the same as ==; that is, it returns true if and only if both variables refer to the same object, if their references are one and the same. Thus you will be testing for object equality and not functional equality.
Always remember to override hashCode if you override equals so as not to "break the contract". As per the API, the result returned from the hashCode() method for two objects must be the same if their equals methods show that they are equivalent. The converse is not necessarily true.

Best Answer

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