Why assertEquals and assertSame in junit return the same result for two instances same class?

Since you didn't override equals in your class, assertEquals behaves the same as assertSame since the default equals implementation compare references.

150    public boolean equals(Object obj) {
151        return (this == obj);
152    }

If you provide a dumb overriding of equals:

class SomeClass {
    @Override 
    public boolean equals(Object o) {
        return true;
    }
}

you'll see that assertEquals succeeds.

assertEquals uses equals() method (that you should override in your class to really compare its instances) to compare objects, while assertSame uses == operator to compare them. So the difference is exactly the same as between == (compare by value) and equals (compare identity).

Official JUnit documentation:

assertEquals: Asserts that two objects are equal.

assertSame: Asserts that two objects refer to the same object.

In other words

assertEquals: uses the equals() method, or if no equals() method was overridden, compares the reference between the 2 objects.

assertSame: compares the reference between the 2 objects.

Example 1: equals method was not overridden, so assertSame and assertEquals return the same result, since they compare the objects' reference.

public class A {    
    private int i;
    public A(int i){ this.i = i; }
}

public class TestA {
    final A a1 = new A(0);
    final A a2 = new A(0);

    @Test
    public void assertsame_testAssertSame(){
        assertSame(a1, a2); // AssertionError: expected:<test2.A@7f13d6e> but was:<test2.A@51cdd8a>
    }

    @Test
    public void assertsame_testAssertEquals(){
        assertEquals(a1, a2); // AssertionError: expected:<test2.A@7f13d6e> but was:<test2.A@51cdd8a>
    }
}

Example 2: equals method was overridden, so assertSame and assertEquals return the not same result, since the equals method will be used by assertEquals this time.

public class A {
    private int i;
    public A(int i){ this.i = i; }

    @Override
    public boolean equals(Object o){
        // self check
        if(this == o){ return true; } else
        // null check
        if(o == null){ return false;} else
        // type check and cast
        if(getClass() != o.getClass()){ return false; } else {
            final A a = (A) o;
            // field comparison
            return Objects.equals(a, a);
        }
    }
}
public class TestA {
    final A a1 = new A(0);
    final A a2 = new A(0);

    @Test
    public void assertsame_testAssertSame(){
        assertSame(a1, a2); // AssertionError: expected:<test2.A@7f13d6e> but was:<test2.A@51cdd8a>
    }

    @Test
    public void assertsame_testAssertEquals(){
        assertEquals(a1, a2); // OK
    }
}