Java Reference
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method main method inc
i = 2000
initially: i = 2000
after i++: i = 2000
location 2000:
FIGURE 7.2: Passing a mutable object to a method.
} public int getValue () {
return i;
} public void setValue( int i) {
this .i=i;
} public String toString() {
return i+ "" ;
Figure 7.2 shows the new configuration. As before, the number 2000 will be passed to
the inc method. The difference now is in the statement: i.setValue(i.getValue()+1) .
Since the i object is mutable, the variable inside the object will be changed from 3 to 4.
Now, when control returns to the main method, the variable i will still be equal to 2000.
However, this time the content at location 2000 has changed from 3 to 4 and therefore the
program will print the number 4.
7.3 The StringBuffer Class
Most immutable classes do not have a mutable version. For example, there is no class
in a Java library that is similar to the MyInteger class. This is intentional because this
will break the rule: “If you pass a variable of type integer to a method, then the calling
method will not see any changes to the variable”. The only exception is the String class.
Sometimes, we want to modify the characters of a String without creating a new String
object. The mutable version of the String class is StringBuffer . Below is an example of
how it can be used.
public class Example {
public static void removeFirstCharacter(StringBuffer s) {
for ( int i=1;i < s . l e n g t h ( ) ; i ++) {
s . setCharAt( i 1, s . charAt( i ) ) ;
s . deleteCharAt(s . length () 1) ;
} public static void main(String [] args)
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