Question

所以我在练习，简单的练习是创建一个Author类，＆amp;一本书课。

他们有正确的getter和setter（测试），但是当我试图获得一本书的作者名称时...它说有一个空对象？（空指针异常）这里是main的摘录：

public static void main(String[] args){
    Author a = new Author("Julius", "j@gmail.com", 'm');
    Book b = new Book("ASDFG", a, 69.99, 4);

    System.out.println(b.getAuthor().getName());
}

我为Author和Book创建实例... 然后我说：得到这本书作者的名字......

我感谢所有人的帮助！

Answer 1

根据您显示的代码，b.getAuthor()返回null。

这意味着您永远不会在Author类中设置Book对象。

你应该有一个看起来像这样的构造函数（完全猜测这些参数）。

private Author author;
// Other fields...

public Book(String title, Author author, double price, int number) {
    this.author = author; // need this
    // other fields = other parameters...
}

您还需要您的getAuthor()方法不返回null

public Author getAuthor() {
    return this.author;
}

Answer 2

您可以添加try {} catch（exception）{}以避免任何异常

我创建子类Book and Author
在作者中，在getname中，我添加了书（书），
添加到作者类： public String getname（）{return＆＃34;作者：＆＃34; +名称+＆＃34;书：＆＃34; + bb.book;}

你可以看到下面的代码：

public class SnackbarManager {
    private static final String TAG = SnackbarManager.class.getName();

    private Snackbar snackbar;
    private Create instance;
    // private boolean isMultiSnackbar;

    public interface Create {
        Snackbar create();
    }

    public SnackbarManager(Create instance) {
        // why not pass in snackbar? coz snackbar.show will fail after 1st show (it multiple snackbar), thus need to recreate it
        snackbar = instance.create();
        this.instance = instance;
    }

    public void show(Fragment fragment) {
        if (fragment.getUserVisibleHint()) {
            snackbar.show();
        }
    }

    public void onSetUserVisibleHint(boolean isVisible) {
        if (isVisible) {
            if (snackbar == null) {
                snackbar = instance.create();
            }
            snackbar.show();
            Log.d(TAG, "showSnackbar="+snackbar.isShown());
            // if snackbar.isShown()=false, if means multiple snackbar exist (might or might not be in same fragment)
            /*
            boolean isMultiSnackbar = !snackbar.isShown();
            // the following is inaccurate when I manually dismiss one of the snackbar
            // even when isShown()=true, the snackbar is not shown
            if (isMultiSnackbar) {
                snackbar = null;
                snackbar = instance.create();
                snackbar.show();
            }
             */
            }
            else {
                Log.d(TAG, "dismissSnackbar");
                snackbar.dismiss();
                // subsequent show will fail, make sure to recreate next
                snackbar = null;
            }
        }
    }
}

public class TestFragment extends Fragment {
    private SnackbarManager snackbarManager;

    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        if (snackbarManager != null)
            snackbarManager.onSetUserVisibleHint(isVisibleToUser);
    }

    public void showSnackbar() {
        snackbarManager = new SnackbarManager(new SnackbarManager.Create() {
            @Override
            public Snackbar create() {
                Snackbar snackbar = Snackbar.make(snackbarLayout, "Create your first moment ;)", Snackbar.LENGTH_INDEFINITE);
                snackbar.setAction("Create", new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        snackbarManager = null;
                    }
                });
                // enable following if not Snackbar.LENGTH_INDEFINITE
                /*
                snackbar.setCallback(new Snackbar.Callback() {
                    @Override
                    public void onDismissed(Snackbar snackbar, int event) {
                        super.onDismissed(snackbar, event);
                        snackbarManager = null;
                    }
                })
                */
                return snackbar;
            }
        });
        snackbarManager.show(this);
    }
}

<强>输出

public class Getauthorname {
    public static void main(String[] args) {
        Author a = new Author("Julius", "j@gmail.com","m");      
        System.out.println(a.getname());
    }
}

class Author{
    Book bb = new Book("ASDFG", "a", 69.99, 4);
    String name;
    String email;
    String m;

    Author( String name,String email, String m){ 
        this.name= name;
        this.email = email;
        this.m = m;
    }

     public void setname(String name){ this.name= name;   }
     //You must add  to get bb.book or other data you need it
     public String getname(){ return "Author :" + name +"  Book:" + bb.book;}

     public void setemail(String email){ this.email= email;   }
     public String getemail(){ return email;}

     public void setm(String m){ this.m= m;   }
     public String getm(){ return m;}
}

class Book extends Object{  

    String book;
    String a;
    double price;
    int cod;

    Book ( String book,String a, double price, int cod){  
        this.book = book;
        this.a = a;
        this.price = price;
        this.cod = cod; 
    }

     public void setbook(String b){ this.book = book;   }
     public String getbook(){ return book;}

     public void seta(String a){ this.a= a;   }
     public String geta(){ return a;}

     public void seta(double price){ this.price= price;   }
     public double getprice(){ return price;}

     public void setcod(int cod){ this.cod= cod;   }
     public int getcod(){ return cod;}
}

从Book-class simple Java获取Author-name

2 个答案: