Android Studio Emulator 在单击按钮时崩溃

Question

我对 Android Studio 非常陌生，我正在尝试创建一个登录屏幕，然后打开一个库存屏幕。 我遇到的问题是当我单击提交到用户名和密码时，应用程序完全崩溃。 现在，我只是在研究如何组织事物的布局部分，但需要弄清楚如何让这种过渡发挥作用。 任何帮助都会很棒！

这是主要的活动代码：

package com.example.cs360projectone;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity {
    EditText username, password;
    Button buttonLogIn, buttonRegister;;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        username = (EditText) findViewById(R.id.username);
        password = (EditText) findViewById(R.id.newPassword);
        buttonLogIn = (Button) findViewById(R.id.buttonLogIn);
        buttonRegister = (Button) findViewById(R.id.buttonRegister);

        buttonLogIn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(username.getText().toString().trim().length() <1 || password.getText().toString().trim().length() <1){
                    username.setError("You must enter a valid username and password");
                }
                else {
                    openInventory();
                }
            }
        });

        buttonRegister.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view1) {

                openRegisterScreen();
            }
        });
    }
    public void openRegisterScreen() {
        Intent intent = new Intent(this, RegisterScreen.class);
        startActivity(intent);
    }
    public void openInventory() {
        Intent intent = new Intent(this, InventoryScreen.class);
        startActivity(intent);
    }
}

这是库存屏幕代码：

package com.example.cs360projectone;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class InventoryScreen extends AppCompatActivity {
    Button buttonSMS, buttonSignOut;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_inventory_screen);

        buttonSMS.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view1) {

                openSMSPermission();
            }
        });
    }
    public void openSMSPermission() {
        Intent intent = new Intent(this, SMSPermissionScreen.class);
        startActivity(intent);
    }
}

这是按下按钮时来自 logcat 的错误：

2022-08-03 16:54:09.037 8043-8043/com.example.cs360projectone E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.cs360projectone, PID: 8043
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.cs360projectone/com.example.cs360projectone.InventoryScreen}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.example.cs360projectone.InventoryScreen.onCreate(InventoryScreen.java:18)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 
2022-08-03 16:54:09.062 8043-8043/com.example.cs360projectone I/Process: Sending signal. PID: 8043 SIG: 9

Answer 1

您需要在 InventoryScreen onCreate 方法中获取 SMS 按钮，方法与使用 MainActivity 中的其他按钮相同。

这意味着做一个

buttonSMS = (Button) findViewById(R.id.buttonSMS);

在设置 onClickListener 之前。