In this blog I will be talking about comparing strings and understanding the String class in detail. As a start string classes are “immutable and final” by nature. These two attributes of the String class go hand-in-hand. If strings have to be immutable, they need be declared final and vice-versa. A final class cannot be overridden so once a method is declared final, they cannot be modified.

The most important thing to understand here is that the JVM memory model for Strings are a bit different from other objects. Strings have a separate memory area called the string constant pool in addition to the heap memory. This helps with immutability of strings.

The following illustration shows why a string reference variable created using ” ” (apostrophe character) is not the same as String created using the ‘new’ keyword. For example –

Let’s say we have two strings –

String s1 = "Hello World !!";
String s2 = new String ("Hello World !!");

We want to compare the s1 and s2 using the == symbol and the equals keyword.

In order to do this I first write a simple StringComparisonDemo class with two methods as shown below and then create unit test to assert expected vs actual results.

public String compareS1EqualsKeywordS2(String s1, String s2) {
    String result = null;   
    if (s1.equals(s2)) {
        result = "equal";
    } else {
        result = "not equal";
    }
    return result;
}
public String compareS1EqualToSymbolsS2(String s1, String s2) {
    String result = null;
    if (s1 == s2) {
        result = "equal";
    } else {
        result = "not equal";
    }
    return result;
}

Unit test #1 – “compareS1EqualsS2″ will compare s1 and s2 values for equality character by character Unit test #2 – “compareS1EqualToSymbolS2” will compare reference variables

@Test 
void compareS1EqualsS2() {
    String expected = "equal";
    StringComparisonDemo demoInstance = new StringComparisonDemo();
    String actual = demoInstance.compareS1EqualsS2(s1, s2);
    assertEquals(expected, actual);
}
@Test
void compareS1EqualToSymbolS2() {
    String expected = "not equal";
    StringComparisonDemo demoInstance = new StringComparisonDemo();
    String actual = demoInstance.compareS1EqualToSymbolsS2(s1, s2);
    assertEquals(expected, actual);
}

I have attached screenshot of the StringComparisonDemo and the Junit test results below.

StringComparisonDemo.java

Junit Result