java - 安卓工作室错误 : E/AndroidRuntime

Question

对于我的一个学校项目，我正在使用 Android Studio 构建自己的应用。这将是我的第一个应用程序。我也是 Android Studio 的新手，刚开始学习 Java。我已经使用 python 构建了一些游戏。

由于我是 Java 和应用程序开发的新手，所以该应用程序非常基础。它为用户提供数学规则和公式，他们可以使用这些规则和公式来研究测试等。

我已经对这个问题进行了研究，我注意到每个人都有不同的代码，这就是为什么我很好奇我是否可以得到一些关于我的代码问题的答案的原因。

我目前在尝试运行我的应用程序时遇到此错误:

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: mcshreker.learnthemalllta, PID: 2876
                  java.lang.RuntimeException: Unable to start activity ComponentInfo{mcshreker.learnthemalllta/mcshreker.learnthemalllta.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.design.widget.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                      at android.app.ActivityThread.-wrap11(ActivityThread.java)
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                      at android.os.Handler.dispatchMessage(Handler.java:102)
                      at android.os.Looper.loop(Looper.java:148)
                      at android.app.ActivityThread.main(ActivityThread.java:5417)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.design.widget.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                      at mcshreker.learnthemalllta.MainActivity.onCreate(MainActivity.java:120)
                      at android.app.Activity.performCreate(Activity.java:6237)
                      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                      at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                      at android.os.Handler.dispatchMessage(Handler.java:102) 
                      at android.os.Looper.loop(Looper.java:148) 
                      at android.app.ActivityThread.main(ActivityThread.java:5417) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

这是我的代码 (MainActivity.Java):

package mcshreker.learnthemalllta;

import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;

import static mcshreker.learnthemalllta.R.layout.activity_main;
import static mcshreker.learnthemalllta.R.layout.activity_main3;
import static mcshreker.learnthemalllta.R.layout.content_main;

@SuppressWarnings("ALL")
public class MainActivity extends AppCompatActivity {

    private Button button;
    public Button button2;
    private Button button4;
    private Button button5;
    public Button button3;
    public Button button7;


    private void init6(){
        button = (Button)findViewById(R.id.button7);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Intent toy6 = new Intent(MainActivity.this,Main8Activity.class);
                startActivity(toy6);
            }
        });
    }

    private void init5(){
        button = (Button)findViewById(R.id.button3);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Intent toy5 = new Intent(MainActivity.this,Main7Activity.class);
                startActivity(toy5);
            }
        });
    }

    private void init4(){
        button = (Button)findViewById(R.id.button2);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Intent toy4 = new Intent(MainActivity.this,Main5Activity.class);
                startActivity(toy4);
            }
        });
    }

    private void init3(){
        button = (Button)findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Intent toy3 = new Intent(MainActivity.this,Main4Activity.class);
                startActivity(toy3);
            }
        });
    }



    private void init2(){
        button5 = (Button)findViewById(R.id.button5);
        button5.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Intent toy2 = new Intent(MainActivity.this,Main3Activity.class);

                startActivity(toy2);
            }
        });
    }

    private void init(){
        button4 = (Button)findViewById(R.id.button4);
        button4.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Intent toy = new Intent(MainActivity.this,Main2Activity.class);

                startActivity(toy);
            }
        });
    }


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        init();
        init2();
        init3();
        init4();
        init5();
        init6();
        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

Answer 1

这可能不是确切的问题，但如果你真的读到了错误，它会告诉你 fab 是空的，这表明你可能删除了存在于你的申请。

尝试删除这部分代码:

FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                .setAction("Action", null).show();
    }
});