Mega Code Archive
A companion class to modify immutable objects
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// : appendixa:Immutable2.java
// A companion class to modify immutable objects.
// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
// www.BruceEckel.com. See copyright notice in CopyRight.txt.
class Mutable {
private int data;
public Mutable(int initVal) {
data = initVal;
}
public Mutable add(int x) {
data += x;
return this;
}
public Mutable multiply(int x) {
data *= x;
return this;
}
public Immutable2 makeImmutable2() {
return new Immutable2(data);
}
}
public class Immutable2 {
private int data;
public Immutable2(int initVal) {
data = initVal;
}
public int read() {
return data;
}
public boolean nonzero() {
return data != 0;
}
public Immutable2 add(int x) {
return new Immutable2(data + x);
}
public Immutable2 multiply(int x) {
return new Immutable2(data * x);
}
public Mutable makeMutable() {
return new Mutable(data);
}
public static Immutable2 modify1(Immutable2 y) {
Immutable2 val = y.add(12);
val = val.multiply(3);
val = val.add(11);
val = val.multiply(2);
return val;
}
// This produces the same result:
public static Immutable2 modify2(Immutable2 y) {
Mutable m = y.makeMutable();
m.add(12).multiply(3).add(11).multiply(2);
return m.makeImmutable2();
}
public static void main(String[] args) {
Immutable2 i2 = new Immutable2(47);
Immutable2 r1 = modify1(i2);
Immutable2 r2 = modify2(i2);
System.out.println("i2 = " + i2.read());
System.out.println("r1 = " + r1.read());
System.out.println("r2 = " + r2.read());
}
} ///:~
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