Java创建对象的六种方法-权当记录一下

是面向对象的编程语言，只要使用它，就需要创建对象。Java创建对象有六种方法，实际常用的不会这么多，这里权当是记录一下。

2 六种方法
（1）使用new关键字

Pumpkin p1 = new Pumpkin();
（2）反射之Class类newInstance()

Pumpkin p2 = Pumpkin.class.newInstance();
（3）反射之Constructor类的newInstance()

Pumpkin p3 = Pumpkin.class.getDeclaredConstructor().newInstance();
（4）Object对象的clone方法

Pumpkin p4 = (Pumpkin) p1.clone();
注意Object类的clone方法是protected的，在Override的时候，可以改成public，这样让其它所有类都可以调用。

注意浅拷贝和深拷贝。

（5）反序列化

ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(“object.bin”));
oos.writeObject(p1);
oos.close();

ObjectInputStream ois = new ObjectInputStream(new FileInputStream(“object.bin”));
Pumpkin p5 = (Pumpkin) ois.readObject();
ois.close();
必须要实现Serializable接口；

需要注意哪些字段可序列化，哪些字段不会被序列化，如何控制；

注意serialVersionUID的作用；

了解Externalizable的不同之处。

（6）使用Unsafe类

Field f = Unsafe.class.getDeclaredField(“theUnsafe”);
f.setAccessible(true);
Unsafe unsafe = (Unsafe) f.get(null);
Pumpkin p6 = (Pumpkin) unsafe.allocateInstance(Pumpkin.class);
很少用的方法，一般不用了解这个方法。

3 示例代码
示例代码如下：

package com.pkslow.basic;

import sun.misc.Unsafe;

import java.io.*;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;

public class CreateObject {

public static class Pumpkin implements Cloneable, Serializable {
    public Pumpkin(){
        System.out.println("Constructor called");
    }
    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

public static void main(String[] args) throws IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, CloneNotSupportedException, IOException, ClassNotFoundException, NoSuchFieldException {

    System.out.println("---start---");
    System.out.println("(1) new");
    Pumpkin p1 = new Pumpkin();

    System.out.println("(2) Class newInstance");
    Pumpkin p2 = Pumpkin.class.newInstance();

    System.out.println("(3) Constructor newInstance");
    Pumpkin p3 = Pumpkin.class.getDeclaredConstructor().newInstance();

    System.out.println("(4) clone");
    Pumpkin p4 = (Pumpkin) p1.clone();

    System.out.println("(5)Serialization");
    ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("object.bin"));
    oos.writeObject(p1);
    oos.close();

    ObjectInputStream ois = new ObjectInputStream(new FileInputStream("object.bin"));
    Pumpkin p5 = (Pumpkin) ois.readObject();
    ois.close();

    System.out.println("(6) Unsafe");
    Field f = Unsafe.class.getDeclaredField("theUnsafe");
    f.setAccessible(true);
    Unsafe unsafe = (Unsafe) f.get(null);
    Pumpkin p6 = (Pumpkin) unsafe.allocateInstance(Pumpkin.class);

    System.out.println("---end---");
}

}
输出结果如下：

—start—
(1) new
Constructor called
(2) Class newInstance
Constructor called
(3) Constructor newInstance
Constructor called
(4) clone
(5)Serialization
(6) Unsafe
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