通过构造函数创建
Java是面向对象的语言，因此创建类实例是Java最重要的一个功能。创建Java对象的时候，构造函数扮演了重要的角色。
隐式构造函数
Java运行定义一个类而无需定义构造函数，这并不意味这构造函数不存在，只是Java编译器会自动的替无构造函数的类生成一个默认无参数的构造函数。例如
package com.javacodegeeks.advanced.construction;
public class NoConstructor {
}无参构造函数
提供无参构造函数相当于Java编译器编译是插入的无参构造函数显示提供
package com.javacodegeeks.advanced.construction;
public class NoArgConstructor {
public NoArgConstructor() {
// Constructor body here
}
}带参数的构造函数
带参数的构造函数是创建实例，初始化最常用的方式，例如
package com.javacodegeeks.advanced.construction;
public class ConstructorWithArguments {
public ConstructorWithArguments(final String arg1,final String arg2) {
// Constructor body here
}
}
final ConstructorWithArguments constructorWithArguments =
new ConstructorWithArguments( "arg1", "arg2" ); 同样的，可以创建多个构造函数，并且构造函数可以互相调用，例如
public ConstructorWithArguments(final String arg1) {
this(arg1, null);
}初始化块
Java 可以通过初始化块来进行初始化，这个特性用的较少，但是仍然需要知道,并且初始化块会在构造函数调用之前调用。
package com.javacodegeeks.advanced.construction;
public class InitializationBlock {
{
// initialization code here
}
}字段默认值
Java对类的字段提供了默认值
- boolean 默认 false
- byte 默认 0
- short 默认 0
- int 默认 0
- long 默认 0L
- char 默认 \u0000
- float 默认 0.0f
- double 默认 0.0d
- Object 默认 null
例如：
package com.javacodegeeks.advanced.construction;
public class InitializationWithDefaults {
private boolean booleanMember;
private byte byteMember;
private short shortMember;
private int intMember;
private long longMember;
private char charMember;
private float floatMember;
private double doubleMember;
private Object referenceMember;
public InitializationWithDefaults() {
System.out.println( "booleanMember = " + booleanMember );
System.out.println( "byteMember = " + byteMember );
System.out.println( "shortMember = " + shortMember );
System.out.println( "intMember = " + intMember );
System.out.println( "longMember = " + longMember );
System.out.println( "charMember = " +
Character.codePointAt( new char[] { charMember }, 0 ) );
System.out.println( "floatMember = " + floatMember );
System.out.println( "doubleMember = " + doubleMember );
System.out.println( "referenceMember = " + referenceMember );
}
}输出
booleanMember = false
byteMember = 0
shortMember = 0
intMember = 0
longMember = 0
charMember = 0
floatMember = 0.0
doubleMember = 0.0
referenceMember = null静态初始化块
相对于初始化块，静态初始化块只是在初始化块前加了static
package com.javacodegeeks.advanced.construction;
public class StaticInitializationBlock {
static {
// static initialization code here
}
} 对象的销毁
对于Java来说，由于有垃圾回收，所以对象创建不再使用后，是有垃圾回收线程默认回收的。但是可以是公用finalizer()方法，来控制垃圾回收。当时先了finalizer()方法后，Java会将其放入Finalizer的垃圾回收队列中。finalizer()是一个很危险的操作，因为它会阻止对象被回收，尤其是操作错误是会导致对象复活，导致垃圾回收失败，所以尽可能不要使用。
创建对象的模式
单例模式
单例模式可以很简单的创建一个对象，并且保证整个应用中仅有一个对象存在。 例如：
package com.javacodegeeks.advanced.construction.patterns;
public class NaiveSingleton {
private static NaiveSingleton instance;
private NaiveSingleton() {
}
public static NaiveSingleton getInstance() {
if( instance == null ) {
instance = new NaiveSingleton();
}
return instance;
}
}或者：
final property of the class.
package com.javacodegeeks.advanced.construction.patterns;
public class EagerSingleton {
private static final EagerSingleton instance = new EagerSingleton();
private EagerSingleton() {
}
public static EagerSingleton getInstance() {
return instance;
}
}或者为了线程安全
package com.javacodegeeks.advanced.construction.patterns;
public class LazySingleton {
private static LazySingleton instance;
private LazySingleton() {
}
public static synchronized LazySingleton getInstance() {
if( instance == null ) {
instance = new LazySingleton();
}
return instance;
}
}当然还可以通过双检锁，静态子类，枚举等方式创建单例
工厂方法
使用工厂方法模式，可以轻松的创建多个对象例如：
package com.javacodegeeks.advanced.construction.patterns;
public class Book {
private Book( final String title) {
}
public static Book newBook( final String title ) {
return new Book( title );
}
}
public interface BookFactory {
Book newBook();
c class Library implements BookFactory {
@Override
public Book newBook() {
return new PaperBook();
}
}
public class KindleLibrary implements BookFactory {
@Override
public Book newBook() {
return new KindleBook();
}
}