空指针异常 怎么办

[qq_35983245]

[合肥-歪歪]

Library L =new Library (); 就行了，或者new Library ().Loan(name);

[weixin_38224089]

[千杯不醉476]

主函数：

public class Borrow {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Book b1 = new Book();
		Book b2 = new Book();
		b1.setBookName("aaa");
		b2.setBookName("bbb");
		Library l = new Library();
		l.setBookArr(new Book[]{b1,b2});
		Student s = new Student();
		s.setName("lucky");
		s.setL(l);
		Book res = s.borrow("bbb");
		if(res!=null){
			System.out.println(s.getName()+":"+res.getBookName());
		}else{
			System.out.println(s.getName()+"no");
		}
	}

}

Book类：

public class Book {
	String BookName;

	public String getBookName() {
		return BookName;
	}

	public void setBookName(String bookName) {
		BookName = bookName;
	}
	
}

Student类：

public class Student {
		private String name;

		public String getName() {
			return name;
		}

		public void setName(String name) {
			this.name = name;
		}
		Library L;
		
		public Library getL() {
			return L;
		}

		public void setL(Library l) {
			L = l;
		}

		public Book borrow(String name){
			return L.load(name);
		}
}

public class Library {

	private Book[] BookArr;
	
	public Book[] getBookArr() {
		return BookArr;
	}

	public void setBookArr(Book[] bookArr) {
		BookArr = bookArr;
	}

	public Book load(String name) {
		// TODO Auto-generated method stub
		for(Book book:this.BookArr)
		if(book.getBookName().equals(name)){
			return book;
		}
		return null;
	}
}

输出结果： 总结：Library是student的属性，要在student里调用libray的方法需要setLibrary

[LateCHI]

L没有初始化。所以报错了。

[seven_day2016]

sudent类的引用没有实例化，所以你就没法直接用l.loan()调方法