Java Automatic Boxing, unpacking

# Java Auto Boxing, unpacking

jdk 1.5starting with the version, this feature is introduced.

First, automatic packing

The base data type is automatically encapsulated as the corresponding wrapper class.

code example,

Integer i = 100;

100is a basic type int and will be boxed automatically, as follows:

Integer i = Integer.valueOf(100);

Equivalent

Integer i = new Integer(100);

Second, automatic unpacking

Automatically converts the encapsulated class to the corresponding base data type.

code example,

Integer i = new Integer(100);int j = i;

iBelongs Integer to the package class and will be automatically disassembled as follows:

Integer i = new Integer(100);int j = i.intValue();

Iii. issues

• Trinocular operator

Problem code (which can cause NPE problems),

Map<String, Boolean> map = new HashMap<String, Boolean>();Boolean b = (map != null ? map.get("test") : false);

Cause: Three mesh operator the second to third operand type is different (one for the object and one for the base data type), and the object is automatically unboxing to the base data type.

Unpacking process,

Map<String, Boolean> map = new HashMap<String, Boolean>();Boolean b = Boolean.valueOf(map ！= null ? map.get("test").booleanValue() : false);

Problem solving,

Map<String, Boolean> map = new HashMap<String, Boolean>();Boolean b = (map != null ? map.get("test") : Boolean.FALSE);

• Integer

Related Code,

Integer int1 = 100;Integer int2 = 100;Integer int3 = 300;Integer int4 = 300;    // trueSystem.out.println(int1 == int2);// falseSystem.out.println(int3 == int4);

Cause the problem: boxing is called Integer.valueOf() . The source code is as follows

public static Integer valueOf(int i) {    assert IntegerCache.high >= 127;    if (i >= IntegerCache.low && i <= IntegerCache.high)        return IntegerCache.cache[i + (-IntegerCache.low)];    return new Integer(i);}

IntegerCacheThe source code is as follows

  private Static class Integercache {static final int low =-128;    static final int high;    Static final Integer cache[];        static {//high value is configured by property int h = 127;        String Integercachehighpropvalue = Sun.misc.VM.getSavedProperty ("Java.lang.Integer.IntegerCache.high");            if (integercachehighpropvalue! = null) {int i = parseint (Integercachehighpropvalue);            i = Math.max (i, 127);        Maximum array size is integer.max_value h = math.min (i, Integer.max_value-(-low)-1);        } high = h;        cache = new Integer[(high-low) + 1];        int j = Low;    for (int k = 0; k < cache.length; k++) cache[k] = new Integer (j + +); Private Integercache () {}}  

No special settings are made and the IntegerCache.high values are 127 . Therefore, Integer.valueOf() when you create an object through a method, Integer if the value is in [-128,127] between, IntegerCache.cache a reference to the object is returned, otherwise a new object is created Integer .

Java Automatic Boxing, unpacking