Java创建对象的几种方法

有时候，也可能碰到这样面试题，如：

Java创建对象有哪几种方法？

除了new之外，java创建对象还有哪几种方式？

本文结合例子，给出几种Java创建对象的方法，Here we go~~~~

使用new创建

这是最常用的一种。如：

Book book = new Book();

示例如下：

package test;

import java.io.Serializable;
import java.util.List;

/**
 * @author wangmengjun
 *
 */
public class Book implements Serializable{

  private static final long serialVersionUID = -6212470156629515269L;

  /**书名*/
  private String name;

  /**作者*/
  private List<String> authors;

  /**ISBN*/
  private String isbn;

  /**价格*/
  private float price;

  public Book() {
  }

  /**
   * @param name
   * @param authors
   * @param isbn
   * @param price
   */
  public Book(String name, List<String> authors, String isbn, float price) {
    this.name = name;
    this.authors = authors;
    this.isbn = isbn;
    this.price = price;
  }

  /**
   * @return the name
   */
  public String getName() {
    return name;
  }

  /**
   * @param name the name to set
   */
  public void setName(String name) {
    this.name = name;
  }

  /**
   * @return the authors
   */
  public List<String> getAuthors() {
    return authors;
  }

  /**
   * @param authors the authors to set
   */
  public void setAuthors(List<String> authors) {
    this.authors = authors;
  }

  /**
   * @return the isbn
   */
  public String getIsbn() {
    return isbn;
  }

  /**
   * @param isbn the isbn to set
   */
  public void setIsbn(String isbn) {
    this.isbn = isbn;
  }

  /**
   * @return the price
   */
  public float getPrice() {
    return price;
  }

  /**
   * @param price the price to set
   */
  public void setPrice(float price) {
    this.price = price;
  }

  /* (non-Javadoc)
   * @see java.lang.Object#toString()
   */
  @Override
  public String toString() {
    return "Book [name=" + name + ", authors=" + authors + ", isbn=" + isbn + ", price="
        + price + "]";
  }

}

    /**
     * 1. 使用new创建对象
     */
    Book book1 = new Book();
    book1.setName("Redis");
    book1.setAuthors(Arrays.asList("Eric", "John"));
    book1.setPrice(59.00f);
    book1.setIsbn("ABBBB-QQ677868686-HSDKHFKHKH-2324234");
    System.out.println(book1);

使用object.clone()

如果要调用clone方法，那么该object需要实现Cloneable接口，并重写clone()方法。

修改后的Book类如下：

package test;

import java.io.Serializable;
import java.util.List;

/**
 * @author wangmengjun
 *
 */
public class Book implements Serializable, Cloneable {

  private static final long serialVersionUID = -6212470156629515269L;

  /**书名*/
  private String name;

  /**作者*/
  private List<String> authors;

  /**ISBN*/
  private String isbn;

  /**价格*/
  private float price;

  public Book() {
  }

  /**
   * @param name
   * @param authors
   * @param isbn
   * @param price
   */
  public Book(String name, List<String> authors, String isbn, float price) {
    this.name = name;
    this.authors = authors;
    this.isbn = isbn;
    this.price = price;
  }

  /**
   * @return the name
   */
  public String getName() {
    return name;
  }

  /**
   * @param name the name to set
   */
  public void setName(String name) {
    this.name = name;
  }

  /**
   * @return the authors
   */
  public List<String> getAuthors() {
    return authors;
  }

  /**
   * @param authors the authors to set
   */
  public void setAuthors(List<String> authors) {
    this.authors = authors;
  }

  /**
   * @return the isbn
   */
  public String getIsbn() {
    return isbn;
  }

  /**
   * @param isbn the isbn to set
   */
  public void setIsbn(String isbn) {
    this.isbn = isbn;
  }

  /**
   * @return the price
   */
  public float getPrice() {
    return price;
  }

  /**
   * @param price the price to set
   */
  public void setPrice(float price) {
    this.price = price;
  }

  /* (non-Javadoc)
   * @see java.lang.Object#toString()
   */
  @Override
  public String toString() {
    return "Book [name=" + name + ", authors=" + authors + ", isbn=" + isbn + ", price="
        + price + "]";
  }

  @Override
  protected Object clone() throws CloneNotSupportedException {
    return (Book) super.clone();
  }

}

测试代码

    /**
     * 1. 使用new创建对象
     */
    Book book1 = new Book();
    book1.setName("Redis");
    book1.setAuthors(Arrays.asList("Eric", "John"));
    book1.setPrice(59.00f);
    book1.setIsbn("ABBBB-QQ677868686-HSDKHFKHKH-2324234");
    System.out.println(book1);

    /**
     * 2. 使用clone创建对象
     */
    try {
      Book book2 = (Book) book1.clone();
      System.out.println(book2);
    } catch (CloneNotSupportedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

使用Class.newInstance()

可以直接使用Class.forName("xxx.xx").newInstance()方法或者XXX.class.newInstance()完成。

    /**
     * 3. 使用Class.newInstance();
     */
    try {
      Book book3 = (Book) Class.forName("test.Book").newInstance();
      System.out.println(book3);

      book3 = Book.class.newInstance();
      System.out.println(book3);
    } catch (InstantiationException | IllegalAccessException | ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

使用Contructor.newInstance()

可以指定构造器来创建，如选择第一个构造器创建；也可以指定构造函数参数类型来创建。

    /**
     * 4. 使用Constructor.newInstance();
     */
    try {
      //选择第一个构造器创建Book
      Book book4 = (Book) Book.class.getConstructors()[0].newInstance();
      //Book [name=null, authors=null, isbn=null, price=0.0]
      System.out.println(book4);

      /**
       * 调用指定构造函数创建对象
       */
      book4 = (Book) Book.class.getConstructor(String.class, List.class, String.class,
          float.class).newInstance("New Instance Example", Arrays.asList("Wang", "Eric"),
          "abc1111111-def-33333", 60.00f);
      //Book [name=New Instance Example, authors=[Wang, Eric], isbn=abc1111111-def-33333, price=60.0]
      System.out.println(book4);
    } catch (InstantiationException | IllegalAccessException | IllegalArgumentException
        | InvocationTargetException | SecurityException | NoSuchMethodException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

使用Class.newInstance()或者Contructor.newInstance()，其本质是一样的，都采用了反射机制。

使用反序列化

    /**
     * 5. 使用反序列化
     */
    try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("book.dat"));
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("book.dat"));) {
      oos.writeObject(book1);

      Book book5 = (Book) ois.readObject();
      System.out.println(book5);

    } catch (IOException | ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

当然了，除了上述几种方式之外，还可以使用JNI等方式来创建对象，这边就不一一列举了。

完整的示例代码如下：

Book.java

package test;

import java.io.Serializable;
import java.util.List;

/**
 * @author wangmengjun
 *
 */
public class Book implements Serializable, Cloneable {

  private static final long serialVersionUID = -6212470156629515269L;

  /**书名*/
  private String name;

  /**作者*/
  private List<String> authors;

  /**ISBN*/
  private String isbn;

  /**价格*/
  private float price;

  public Book() {
  }

  /**
   * @param name
   * @param authors
   * @param isbn
   * @param price
   */
  public Book(String name, List<String> authors, String isbn, float price) {
    this.name = name;
    this.authors = authors;
    this.isbn = isbn;
    this.price = price;
  }

  /**
   * @return the name
   */
  public String getName() {
    return name;
  }

  /**
   * @param name the name to set
   */
  public void setName(String name) {
    this.name = name;
  }

  /**
   * @return the authors
   */
  public List<String> getAuthors() {
    return authors;
  }

  /**
   * @param authors the authors to set
   */
  public void setAuthors(List<String> authors) {
    this.authors = authors;
  }

  /**
   * @return the isbn
   */
  public String getIsbn() {
    return isbn;
  }

  /**
   * @param isbn the isbn to set
   */
  public void setIsbn(String isbn) {
    this.isbn = isbn;
  }

  /**
   * @return the price
   */
  public float getPrice() {
    return price;
  }

  /**
   * @param price the price to set
   */
  public void setPrice(float price) {
    this.price = price;
  }

  /* (non-Javadoc)
   * @see java.lang.Object#toString()
   */
  @Override
  public String toString() {
    return "Book [name=" + name + ", authors=" + authors + ", isbn=" + isbn + ", price="
        + price + "]";
  }

  @Override
  protected Object clone() throws CloneNotSupportedException {
    return (Book) super.clone();
  }

}

CreateObjectExample.java

package test;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.InvocationTargetException;
import java.util.Arrays;
import java.util.List;

/**
 * @author wangmengjun
 *
 */
public class CreateObjectExample {

  public static void main(String[] args) {
    /**
     * 1. 使用new创建对象
     */
    Book book1 = new Book();
    book1.setName("Redis");
    book1.setAuthors(Arrays.asList("Eric", "John"));
    book1.setPrice(59.00f);
    book1.setIsbn("ABBBB-QQ677868686-HSDKHFKHKH-2324234");
    System.out.println(book1);

    /**
     * 2. 使用clone创建对象
     */
    try {
      Book book2 = (Book) book1.clone();
      System.out.println(book2);
    } catch (CloneNotSupportedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

    /**
     * 3. 使用Class.newInstance();
     */
    try {
      Book book3 = (Book) Class.forName("test.Book").newInstance();
      System.out.println(book3);

      book3 = Book.class.newInstance();
      System.out.println(book3);
    } catch (InstantiationException | IllegalAccessException | ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

    /**
     * 4. 使用Constructor.newInstance();
     */
    try {
      //选择第一个构造器创建Book
      Book book4 = (Book) Book.class.getConstructors()[0].newInstance();
      //Book [name=null, authors=null, isbn=null, price=0.0]
      System.out.println(book4);

      /**
       * 调用指定构造函数创建对象
       */
      book4 = (Book) Book.class.getConstructor(String.class, List.class, String.class,
          float.class).newInstance("New Instance Example", Arrays.asList("Wang", "Eric"),
          "abc1111111-def-33333", 60.00f);
      //Book [name=New Instance Example, authors=[Wang, Eric], isbn=abc1111111-def-33333, price=60.0]
      System.out.println(book4);
    } catch (InstantiationException | IllegalAccessException | IllegalArgumentException
        | InvocationTargetException | SecurityException | NoSuchMethodException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

    /**
     * 5. 使用反序列化
     */
    try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("book.dat"));
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("book.dat"));) {
      oos.writeObject(book1);

      Book book5 = (Book) ois.readObject();
      System.out.println(book5);

    } catch (IOException | ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
  }

}

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