Java Basics (a) the object wrapper

Object wrappers

Each type has a corresponding base class called wrapper warpper .

/* extends Number */
// Interger         <-> int
// Long         <-> long
// Float        <-> float
// double       <-> double
// Short        <-> short
// Byte         <-> byte

/* not extends Number */
// Character        <-> character
// Void         <-> void
// Boolean      <-> boolean

Packaging the object is immutable : once the configuration of the wrapper, the packaging is not allowed to change the value therein.

Object wrapper are final: the definition of the object can not be a subclass of the wrapper.

Autoboxing (auto boxing)

Declare an Integerarray list object and add a value of 3 Integerobjects.

// 由于值包装在对象中，ArrayList<Integer>的效率远低于int[]数组。
ArrayList<Integer> list = new ArrayList<>();
// 等价于list.add(Integer.valueOf(3));
list.add(3);

On the contrary, the Integerobject is assigned to intthe time variable, it will automatically unpack .

// 编译器将以下语句翻译成int n = list.get(i).inValue();
int n = list.get(i);

It can be automatically boxing and unboxing in arithmetic expressions.

Integer n = 3;
// 编译器会自动地插入一条对象拆箱指令，在自增计算执行后，执行装箱指令。
n++;

Compare wrapper

Object wrapper equality with different basic types:

Integer a = 1000;
Integer b = 1000;
// ==检测a和b是否指向同一个存储区域，结果是false
System.out.println(a == b);
// equals方法比较a和b包装的值，结果是true
System.out.println(a.equals(b));

There are automatic packing specifications of ranges of values basic types: boolean, byte, char≤ 127 , between -128 to 127 between shortand intare packed in a fixed object, which points to the same memory area.

Integer a = 100;
Integer b = 100;
// 此时a和b指向同一个存储区域，结果是true
System.out.println(a == b);

Abnormal null reference

Packaging Cite null, autoboxing may throw NullPointerExceptionan exception:

Integer n = null;
// 由于n引用null，执行2 * n时会将n自动拆箱取值，抛出NullPointerException异常。
System.out.println(2 * n);

auto update

Further mixed using conditional expressions Integerand Doublewhen the Integervalue unpacking promoted double, then packing as Double:

Integer n = 1;
Double x = 2.0;
// n升级为Double，结果是1.0
System.out.println(true ? n : x);

Note : boxing and unboxing is a compiler recognized, instead of the virtual machine. The compiler generates bytecode classes and then, inserts the necessary method call.