Un Modifiable Wrapper Example in Java

Introduction

Un Modifiable Wrapper is a classic Java console program that demonstrates the concept with complete source code and sample output. The Collections Framework provides ArrayList, HashMap, HashSet and related data structures.

This tutorial walks through the program line by line, explains how the logic works, and highlights best practices you can apply in your own code.

Definition

A wrapper class is a class that encapsulates types, so that those types can be used to create object instances and methods in another class that need those types. An UnModifiableWrapper is a wrapper which cannot be modified. It produces UnsupportedOperationException when tried to edit.

Syntax

Collections.unmodifiableList(list-name)
Collections.unmodifiableSet(set-name)
Collections.unmodifiableMap(map-name)

Un Modifiable Wrapper Example Program

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class UnmodifiableWrappersExample {

    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<String>();

        list.add("Hundred");
        list.add("Two Hundred");
        list.add("Three Hundred");
        list.add("Four Hundred");
        list.add("Five Hundred");

        System.out.println("Initial value of list : " + list);

        List<String> unModifiableList = Collections.unmodifiableList(list);
        System.out.println("Values of Unmodifiable list : " + unModifiableList);
        unModifiableList.add("Thousand");// Produces Exception
        System.out.println("After adding value 'Thousand' to list, the value is : " + unModifiableList);
    }
}

Sample Output

Initial value of list : [Hundred, Two Hundred, Three Hundred, Four Hundred, Five Hundred]
Values of Unmodifiable list : [Hundred, Two Hundred, Three Hundred, Four Hundred, Five Hundred]
Exception in thread "main" java.lang.UnsupportedOperationException
        at java.util.Collections$UnmodifiableCollection.add(Collections.java:1055)
        at UnmodifiableWrappersExample.main(UnmodifiableWrappersExample.java:21)

When to use

Use this un modifiable wrapper example when learning or revising core Java syntax.

How it works

1. Execution begins in the main method — the JVM calls this method when you run the class.

2. import java.util.ArrayList; imports a class used later in the program.

3. import java.util.Collections; imports a class used later in the program.

4. import java.util.List; imports a class used later in the program.

5. ArrayList<String> list = new ArrayList<String>(); updates a variable used in the calculation or output.

6. A println / print call writes text to the console — part of the sample output below.

7. List<String> unModifiableList = Collections.unmodifiableList(list); updates a variable used in the calculation or output.

8. A println / print call writes text to the console — part of the sample output below.

Best Practices

• Use meaningful variable and class names that describe their purpose.
• Compile and run the program locally — modify values to see how output changes.
• Read compiler errors carefully; they usually point to the exact line to fix.

Common Mistakes

• Copying code without understanding each line — practice by changing one statement at a time.
• Mismatching the public class name and the .java filename.
• Forgetting semicolons at the end of statements.