【转载】创建对象的5种方法

创建对象的方式	是否调用了构造器
new关键字	是
Class类的newInstance方法	是
Constructor类的newInstance方法	是
Clone方法	否
反序列化	否

测试类Teacher

import java.io.Serializable;
public class Teacher implements Serializable{
    private static final long serialVersionUID = 8363559040208136513L;
    public Teacher() {
        System.out.println("老师的无参构造方法");
        // TODO Auto-generated constructor stub
    } 
}

测试类Student

import java.io.Serializable;
import java.util.Date;

public class Student implements Cloneable,Serializable{
    private static final long serialVersionUID = -3347859007645821784L;
    private Long id;
        private String name;
        private Integer age;
        private Date birthday;
        private Teacher teacher;
   
    public Student() {
        System.out.println("默认构造方法");
    }
    
    public Student(Long id) {
        System.out.println("id构造方法");
        this.id = id;
    }

    public Student(Long id, String name, Integer age, Date birthday, Teacher teacher) {
        System.out.println("全属性构造方法");
        this.id = id;
        this.name = name;
        this.age = age;
        this.birthday = birthday;
        this.teacher = teacher;
    }
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
    public Date getBirthday() {
        return birthday;
    }
    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }
    public Teacher getTeacher() {
        return teacher;
    }
    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }
    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + ", age=" + age + ", birthday=" + birthday + ", teacher="
                + teacher + "]";
    }
    @Override
    protected Object clone() throws CloneNotSupportedException {
        // TODO Auto-generated method stub
        return super.clone();
    }

}

   一、new关键字创建对象，是最常见的创建对象的方式，可以调用任意public构造函数

Student student1 = new Student(100L);
System.out.println(student1);

  二、 使用Class类的newInstance方法，只能调用默认的无参构造器，其实本质上是调用反射相关类Constructor类的无参newInstance方法。

Student student2 = Student.class.newInstance();
System.out.println(student2);

 三、使用Constructor类的newInstance方法，可以调用任意构造器，包括有参无参，公有非公有

Constructor<Student> constructor = Student.class.getConstructor();
Student student3 = constructor.newInstance();
Constructor<Student> constructor1 = Student.class.getConstructor(Long.class);
Student student4 = constructor1.newInstance(100L);

 四、使用Object的clone方法，不会调用任何构造函数，需要实现Cloneable接口，native本地方法实现,JVM新建一个对象，将原对象的内容全部拷贝进去，如果原对象内属性有引用对象，则新的对象的对应属性仍指向该引用对象。

Student student5 = (Student)student1.clone();
student5.setTeacher(new Teacher());
System.out.println(student5.getTeacher().hashCode());
Student student6 = (Student) student5.clone();
System.out.println("student6:"+student6.getTeacher().hashCode());

 五、使用反序列化，需要实现Serializable，不调用任何构造器,如果里面属性信息有引用对象，反序列化出的对象内的对应属性引用对象会重新创建（仍然不调用构造器）

        ObjectOutputStream oos = null;
        ByteArrayOutputStream baos = null;
        ObjectInputStream ois = null;
        ByteArrayInputStream bais = null;
        byte[] bs = null;
        
        try {
            //序列化
            baos = new ByteArrayOutputStream();
            oos = new ObjectOutputStream(baos);
            oos.writeObject(student6);
            oos.flush();
            bs = baos.toByteArray();
            
            
            //反序列化
            bais = new ByteArrayInputStream(bs);
            ois = new ObjectInputStream(bais);
            Student student7 = (Student) ois.readObject();
            System.out.println("student7:"+student7.hashCode());
            System.out.println("student7:"+student7.getTeacher().hashCode());
            
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

 下面列出测试类的全部代码：

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class CreateObject {
    public static void main(String[] args) throws InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException, CloneNotSupportedException {
        Student student1 = new Student(100L);
        System.out.println(student1);
        
        Student student2 = Student.class.newInstance();
        System.out.println(student2);
        
        Constructor<Student> constructor = Student.class.getConstructor();
        Student student3 = constructor.newInstance();
        Constructor<Student> constructor1 = Student.class.getConstructor(Long.class);
        Student student4 = constructor1.newInstance(100L);
        
        Student student5 = (Student)student1.clone();
        student5.setTeacher(new Teacher());
        System.out.println(student5.getTeacher().hashCode());
        Student student6 = (Student) student5.clone();
        System.out.println("student6:"+student6.getTeacher().hashCode());
        
        
        ObjectOutputStream oos = null;
        ByteArrayOutputStream baos = null;
        ObjectInputStream ois = null;
        ByteArrayInputStream bais = null;
        byte[] bs = null;
        
        try {
            //序列化
            baos = new ByteArrayOutputStream();
            oos = new ObjectOutputStream(baos);
            oos.writeObject(student6);
            oos.flush();
            bs = baos.toByteArray();
            
            
            //反序列化
            bais = new ByteArrayInputStream(bs);
            ois = new ObjectInputStream(bais);
            Student student7 = (Student) ois.readObject();
            System.out.println("student7:"+student7.hashCode());
            System.out.println("student7:"+student7.getTeacher().hashCode());
            
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}