Wrapper Classes & Autoboxing

Syntax

WrapperClass variable = primitiveValue; // autoboxing

Examples

Integer boxedInt = 42;
Double boxedDouble = 3.14;
Boolean boxedBoolean = true;
Character boxedChar = 'A';

System.out.println(boxedInt);
System.out.println(boxedDouble);

int primitive = 10;
Integer boxed = primitive;    // autoboxing: int -> Integer

Integer wrapped = 25;
int unboxed = wrapped;         // unboxing: Integer -> int

System.out.println(boxed + unboxed);  // 35, works seamlessly

import java.util.ArrayList;

// ArrayList<int> numbers = new ArrayList<>();  // Error! primitives not allowed

ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(10);   // autoboxed to Integer automatically
numbers.add(20);

int first = numbers.get(0);  // auto-unboxed back to int
System.out.println(first);

String numText = "123";
int parsed = Integer.parseInt(numText);
System.out.println(parsed + 1);  // 124

System.out.println(Integer.MAX_VALUE);  // 2147483647
System.out.println(Integer.MIN_VALUE);  // -2147483648

System.out.println(Integer.toBinaryString(10));  // 1010

Best practices

• Use wrapper classes when working with generics and collections (ArrayList<Integer>, not ArrayList<int>), since generics require object types
• Prefer primitives over wrapper classes for simple local computation - they avoid the small overhead of object creation and unboxing
• Be careful comparing wrapper objects with == - like String, it compares references, not values, outside a small cached range (-128 to 127 for Integer)
• Watch for a NullPointerException when unboxing a wrapper object that is null - a null Integer cannot be auto-unboxed to int

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation