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,包括:
Book book = new Book();
编写一个Book类
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 + "]"; }
}
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);
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(); }
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(); }
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。
五、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。