java List深拷贝的两种方式

大家都知道java里面分为浅拷贝和深拷贝。举个简单的例子，区分一下浅拷贝和深拷贝的区别

public class Address{

    private String address;

    public Address(String address){
        this.address = address;
    }

    public String getAddress(){
        return address;
    }

    public void setAddress(String address){
        this.address = address;
    }

    @Override
    protected Object clone() {
        Address address = null;
        try {
            address = (Address) super.clone();
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }
        return address;
    }
}

public class Student implements Cloneable{

	private String name;
	public List<Address> addressList;

	public Student(){
    }

	public Student(String name) {
		this.name = name;
	}

	public void setName(String name){
		this.name = name;
	}

	public String getName() {
		return name;
	}

	@Override
	protected Object clone() {
		Student student = null;
		try {
			student = (Student) super.clone();
		} catch (CloneNotSupportedException e) {
			e.printStackTrace();
		}
		return student;
	}
}

先创建两个类，一个是地址，一个是学生，便于后边的打印结果能明显区分浅拷贝和深拷贝。

public class myClass {

	public static void main(String[] args) {
		Student student = new Student("李晓东");
		Student student2 = (Student) student.clone();

		println(student.getName() );
		println(student2.getName() );
		println("改变student2的姓名后--------------");
		student2.setName("张天");
		println(student.getName() );
		println(student2.getName());
	}

	public static void println(String str) {
		System.out.println(str);
	}


}

打印结果如下

李晓东
李晓东
改变student2的姓名后--------------
李晓东
张天

可以看到当我们改变student2的name值的时候，并没有改变student的name值(String在此时不属于引用值类型)
在我们的项目当中，经常会遇到一个类里面有List，然后List装载另外一个对象，这个时候要进行深拷贝就需要用到如下的办法，先把Address进行序列化和实现cloneable接口并且重写clone方法。

public class Address implements Cloneable,Serializable{

    private String address;

    public Address(String address){
        this.address = address;
    }

    public String getAddress(){
        return address;
    }

    public void setAddress(String address){
        this.address = address;
    }

    @Override
    protected Object clone() {
        Address address = null;
        try {
            address = (Address) super.clone();
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }
        return address;
    }
}

public class myClass {

	public static void main(String[] args) {
		List<Address> addressList = new ArrayList<>();
		addressList.add(new Address("北京市"));
		Student student = new Student("李晓东");
		student.addressList = addressList;

		Student student2 = (Student) student.clone();
		student2.addressList = depCopy2(addressList);

		println(student.getName() + "---" + student.addressList.get(0).getAddress());
		println(student2.getName() + "---" + student2.addressList.get(0).getAddress());
		println("改变student2的姓名后--------------");
		student2.setName("张天");
		student2.addressList.get(0).setAddress("湖南省");
		println(student.getName() + "---" + student.addressList.get(0).getAddress());
		println(student2.getName() + "---" + student2.addressList.get(0).getAddress());
	}

	public static void println(String str) {
		System.out.println(str);
	}

	/***
	 * 方法二
	 * 需要Address实现cloneable接口和重写clone方法，次方法有限制性，
	 * 例如要先声明List是保存的什么对象，并且当碰到对象里面还持有List集合的时候
	 * 就不管用的，所以建议使用第一种方法
	 * 
	 * @param addresses
	 * @return
	 */
	public static List<Address> depCopy2(List<Address> addresses) {
		List<Address> destList = new ArrayList<>();
		for (Address address : addresses) {
			destList.add((Address) address.clone());
		}
		return destList;
	}

	/***
	 * 方法一对集合进行深拷贝 注意需要对泛型类进行序列化(实现Serializable)
	 * 
	 * @param srcList
	 * @param <T>
	 * @return
	 */
	public static <T> List<T> depCopy(List<T> srcList) {
		ByteArrayOutputStream byteOut = new ByteArrayOutputStream();
		try {
			ObjectOutputStream out = new ObjectOutputStream(byteOut);
			out.writeObject(srcList);

			ByteArrayInputStream byteIn = new ByteArrayInputStream(byteOut.toByteArray());
			ObjectInputStream inStream = new ObjectInputStream(byteIn);
			List<T> destList = (List<T>) inStream.readObject();
			return destList;
		} catch (IOException e) {
			e.printStackTrace();
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		}
		return null;
	}

}

此时打印结果是

李晓东---北京市
李晓东---北京市
改变student2的姓名后--------------
李晓东---北京市
张天---湖南省

如果没看明白，可以自己修改不实现cloneable接口和注释掉clone方法再执行看结果，一定要自己写自己试，这样才会印象深刻!!!