Classes and Objects - Beginning Java 8 Fundamentals - page 239

What will be the content of s1 at #4? String is a reference type in Java. At #1, s1 is referencing a String object
whose content is "hi" . When the changeString(s1) method is called, s1 is passed to s2 by reference value. At #2,
s1 and s2 are referencing the same String object in memory whose content is "hi". When s2 = s2 + " there"
statement is executed, two things happens. First, s2 + " there" expression is evaluated, which creates a new String
object in memory with content of "hi there" returns its reference. The reference returned by the s2 + " there"
expression is assigned to s2 formal parameter. At this time, there are two String objects in memory: one with the
content of "hi" and another with the content of "hi there" . At #3, the actual parameter s1 is referencing the String
object with the content of "hi" and the formal parameter s2 is referencing the String object with content "hi there" .
When the changeString() method call is over, the formal parameter s2 is discarded. Note that the String object with
content "hi there" still exists in memory after the changeString() method call is over. Only the formal parameter
is discarded when a method call is over and not the object to which the formal parameter was referencing. At #4,
the reference variable s1 still refers to the String object with content "hi" . Listing 6-25 has the complete code that
attempts to modify a formal parameter of String type.

Listing 6-25. Another Example of Pass by Reference Value Parameter Passing in Java

// PassByReferenceValueTest2.java
package com.jdojo.cls;

public class PassByReferenceValueTest2 {
public static void changeString(String s2) {
System.out.println("#2: s2 = " + s2);
s2 = s2 + " there";
System.out.println("#3: s2 = " + s2);
}

public static void main(String[] args) {
String s1 = "hi";
System.out.println("#1: s1 = " + s1);
PassByReferenceValueTest2.changeString(s1);
System.out.println("#4: s1 = " + s1);
}
}

#1: s1 = hi
#2: s2 = hi
#3: s2 = hi there
#4: s1 = hi

■ a String object is immutable, meaning that its content cannot be changed after it is created. If you need to
change the content of a String object, you must create a new String object with the new content.

Tip