This is an example of how to use final fields in a class, so as to understand the difference between final and non final fields in a class. In short the example is described below:
- We have created a class
FinalFieldsthat hasfinalattributes, such as typical public constants and compile-time constants (private). - We create a new instance of
FinalFields. - We can access and change the attributes of
FinalFieldsthat are notfinal, but we cannot changefinalattributes. - For example, we can change the
a1field, and set it to a new instance ofAclass, sinceAclass is notfinal. - We can also change the values of the int fields in the
finalarray, but we cannot change the array and set it to a new int array with different length.
Let’s take a look at the code snippet that follows:
package com.javacodegeeks.snippets.core;
import java.util.Random;
class A {
int i; // Package access
public A(int i) {
this.i = i;
}
}
public class FinalFields {
private static Random rand = new Random();
private String str;
public FinalFields(String str) {
this.str = str;
}
// Can be compile-time constants:
private final int ONE = 9;
private static final int TWO = 99;
// Typical public constant:
public static final int THREE = 39;
// Cannot be compile-time constants:
private final int X = rand.nextInt(20);
static final int Y = rand.nextInt(20);
private A a1 = new A(11);
private final A a2 = new A(22);
private static final A a3 = new A(33);
// Arrays:
private final int[] array = {1, 2, 3, 4, 5, 6};
@Override
public String toString() {
return str + ": " + "i4 = " + X + ", i5 = " + Y;
}
public static void main(String[] args) {
FinalFields fd1 = new FinalFields("fd1");
//! fd1.ONE++; // Error: can't change value
fd1.a2.i++; // Object isn't constant!
fd1.a1 = new A(9); // OK -- not final
for (int i = 0; i < fd1.array.length; i++) {
fd1.array[i]++; // Object isn't constant!
} //! fd1.v2 = new Value(0); // Error: Can't
//! fd1.v3 = new Value(1); // change reference
//! fd1.a = new int[3];
System.out.println(fd1);
System.out.println("Creating new FinalFields");
FinalFields fd2 = new FinalFields("fd2");
System.out.println(fd1);
System.out.println(fd2);
}
}
Output:
fd1: X = 15, Y = 8
Creating new FinalFields
fd1: X = 15, Y = 8
fd2: X = 1, Y = 8
This was an example of how to use final fields in a class in Java.
