Java建立物件的幾種方法

有時候，也可能碰到這樣面試題，如：

Java創建物件有哪幾種方法？

除了new之外，java建立物件還有哪幾種方式？

本文結合例子，給出幾種Java創建物件的方法，Here we go~~~~

使用new創建

這是最常用的一種。如：

Book book = new Book();

範例如下：

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()

如果要呼叫clone方法，那麼該object需要實作Cloneable介面，並重寫clone方法方法。

修改後的Book類別如下：

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();
    }

}

使用Class.newInstance()

可以直接使用Class.forName("xxx.xx").newInstance()方法或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()

可以指定建構器來創建，例如選擇第一個建構器創建；也可以指定建構子參數類型來建立。

      /**
         * 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Contructor.newInstance()，其本質是相同的，都採用了反射機制。

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();
        }

當然了，除了上述幾種方式之外，還可以使用JNI等方式來創建對象，這邊就不一一列舉了。

完整的範例程式碼如下：

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();
        }
    }

}