java我如何在安卓 studio中为电子商务应用解决这个问题？

Question

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.shopingonline, PID: 18327
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.shopingonline/com.example.shopingonline.HomeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.shopingonline.Model.Users.getName()' on a null object reference

View headerView = navigationView.getHeaderView(0);
        TextView userNameTextView = headerView.findViewById(R.id.user_profile_name);
        CircleImageView profileImageView = headerView.findViewById(R.id.user_profile_image);

        userNameTextView.setText(Prevalent.currentOnlineUser.getName());

public class Prevalent {
    public static Users currentOnlineUser;
    //unique key for all users
    public static final String UserPhoneKey = "UserPhone";
    public static final String UserPasswordKey = "UserPassword";
}



public class Users {
    //get the name passeword, all the user information
    private String name, phone, password;

    public Users(String name, String phone, String password) {
        this.name = name;
        this.phone= phone;
        this.password = password;
    }

    public Users(){

    };

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone;

    } public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
}

//我有这个问题，我解决不了。我试了4天，但对初学者来说太难了Gjvkhkbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

Answer 1

从您提到的异常可以看出，您正在访问一个null引用上的方法getName()，该引用阻止您启动活动

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.shopingonline, PID: 18327 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.shopingonline/com.example.shopingonline.HomeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.shopingonline.Model.Users.getName()' on a null object reference

您首先需要检查是否需要空引用，如果需要，您可以添加空检查以避免它。有关详细信息，请参见this

if(Prevalent.currentOnlineUser != null)
    userNameTextView.setText(Prevalent.currentOnlineUser.getName());