java.lang.NullPointerException:尝试调用虚拟方法android [英] java.lang.NullPointerException: Attempt to invoke virtual method android
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问题描述
我是android的初学者，我不知道代码中的问题是什么！我需要您的帮助，这是我的代码:我正在尝试做一个计算器=-/* .....，例如，我删除了if view == pbtn ...只是为了尝试onclick函数...而我找不到我的问题
i am a beginner in the android, and i dont know what is the problem in my code ! and i need your help, this is my code: i am trying to do a calculator = - / * ..... and i removed the if view==pbtn for example...just to try the onclick function...and i cant find what is my problem
package com.michael.mikematta.calculator;
import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import static com.michael.mikematta.calculator.R.id.FirstNum;
import static com.michael.mikematta.calculator.R.id.HelokBtn;
import static com.michael.mikematta.calculator.R.id.KafolBtn;
import static com.michael.mikematta.calculator.R.id.MinusBtn;
import static com.michael.mikematta.calculator.R.id.PlusBtn;
import static com.michael.mikematta.calculator.R.id.SecondNum;
import static com.michael.mikematta.calculator.R.id.TheResult;
public class MainActivity extends Activity implements View.OnClickListener {
EditText firstnumx;
EditText secondnum;
Button pbtn;
Button minusbtn;
Button kafolbtn;
Button helokbtn;
TextView theresult;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button pbtn = (Button) findViewById(R.id.PlusBtn);
Button minusbtn = (Button) findViewById(R.id.MinusBtn);
Button kafolbtn = (Button) findViewById(R.id.KafolBtn);
Button helokbtn = (Button) findViewById(R.id.HelokBtn);
EditText firstnum = (EditText) findViewById(R.id.FirstNum);
EditText secondnum = (EditText) findViewById(R.id.SecondNum);
TextView theresult = (TextView) findViewById(R.id.TheResult);
minusbtn.setOnClickListener(this);
pbtn.setOnClickListener(this);
kafolbtn.setOnClickListener(this);
helokbtn.setOnClickListener(this);
}
@Override
public void onClick(View view) {
theresult.setText("hi");
}
}
我的xml:
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="@drawable/ic_launcher"
android:orientation="vertical"
tools:context=".MainActivity">
<TextView
android:text="Hello Calculator"
android:layout_width="match_parent"
android:layout_height="50dp"
android:gravity="center"
android:textSize="20dp"
android:textColor="#000"
android:textStyle="bold"
/>
<EditText
android:layout_width="200dp"
android:layout_height="50dp"
android:layout_gravity="center"
android:gravity="center"
android:hint="Enter First Num"
android:background="#fff"
android:id="@+id/FirstNum"
android:layout_margin="10dp"
/>
<EditText
android:layout_width="200dp"
android:layout_height="50dp"
android:layout_gravity="center"
android:gravity="center"
android:hint="Enter Second Num"
android:background="#fff"
android:id="@+id/SecondNum"
android:layout_margin="10dp"
/>
<Button
android:layout_width="200dp"
android:layout_height="50dp"
android:layout_gravity="center"
android:gravity="center"
android:text="+"
android:background="#ffeeeeee"
android:textSize="22dp"
android:id="@+id/PlusBtn"
android:layout_margin="10dp"
/>
<Button
android:layout_width="200dp"
android:layout_height="50dp"
android:layout_gravity="center"
android:gravity="center"
android:text="-"
android:background="#ffeeeeee"
android:textSize="22dp"
android:id="@+id/MinusBtn"
android:layout_margin="10dp"
/>
<Button
android:layout_width="200dp"
android:layout_height="50dp"
android:layout_gravity="center"
android:gravity="center"
android:text="*"
android:background="#ffeeeeee"
android:textSize="22dp"
android:id="@+id/KafolBtn"
android:layout_margin="10dp"
/>
<Button
android:layout_width="200dp"
android:layout_height="50dp"
android:layout_gravity="center"
android:gravity="center"
android:text="/"
android:background="#ffeeeeee"
android:textSize="22dp"
android:id="@+id/HelokBtn"
android:layout_margin="10dp"
/>
<TextView
android:text="Here will be your result"
android:layout_width="match_parent"
android:layout_height="50dp"
android:gravity="center"
android:textSize="20dp"
android:textColor="#fff"
android:textStyle="bold"
android:background="#ffff714f"
android:id="@+id/TheResult"
/>
</LinearLayout>
例外:
02-08 03:09:50.468 1648-1666/com.michael.mikematta.calcu2 W/EGL_emulation﹕ eglSurfaceAttrib not implemented
02-08 03:09:50.468 1648-1666/com.michael.mikematta.calcu2 W/OpenGLRenderer﹕ Failed to set EGL_SWAP_BEHAVIOR on surface 0xaeeff540, error=EGL_SUCCESS
02-08 03:09:50.538 1648-1648/com.michael.mikematta.calcu2 I/Choreographer﹕ Skipped 33 frames! The application may be doing too much work on its main thread.
02-08 03:09:51.177 1648-1648/com.michael.mikematta.calcu2 I/Choreographer﹕ Skipped 37 frames! The application may be doing too much work on its main thread.
02-08 03:09:53.984 1648-1648/com.michael.mikematta.calcu2 D/AndroidRuntime﹕ Shutting down VM
02-08 03:09:54.000 1648-1648/com.michael.mikematta.calcu2 E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.michael.mikematta.calcu2, PID: 1648
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setText(java.lang.CharSequence)' on a null object reference
at com.michael.mikematta.calcu2.MainActivity.onClick(MainActivity.java:47)
at android.view.View.performClick(View.java:4756)
at android.view.View$PerformClick.run(View.java:19749)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
02-08 03:09:57.267 1648-1648/com.michael.mikematta.calcu2 I/Process﹕ Sending signal. PID: 1648 SIG: 9
有什么帮助吗?
推荐答案
您正在初始化 onCreate 中的变量 theresult 的本地副本:
You are initializing a local copy of variable theresult in onCreate:
TextView theresult = (TextView) findViewById(R.id.TheResult);
但是在 onClick 中，您尝试访问变量 theresult 的另一个实例，该实例在 onCreate 之外声明.这个尚未被初始化.这就是为什么在尝试执行 setText 时给其null的原因.
But in onClick you are trying to access another instance of the variable theresult which is declared outside onCreate. This one has not been initialized yet. That's why giving its giving null when you are trying to do setText.
要解决，请替换以下行:
To solve, replace the below line:
TextView theresult = (TextView) findViewById(R.id.TheResult);
使用
theresult = (TextView) findViewById(R.id.TheResult);
这篇关于java.lang.NullPointerException:尝试调用虚拟方法android的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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