JJOE64 GraphView Library

Question

嗨我想通過JJOE64使用GraphView庫，我在我的項目中使用Eclipse「添加外部檔案」導入.jar文件。

我創建了一個新的GrapVew對象

package com.example.graphtest; 

import android.os.Bundle; 
import android.app.Activity; 
import android.view.Menu; 
import com.jjoe64.graphview.*; 

public class MainActivity extends Activity { 

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


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

    GraphView Grafico = new LineGraphView(this, "GraphTest"); 


} 

和我扮演的應用，看看會發生什麼，我得到這個致命的錯誤

03-03 15:22:11.863: E/Trace(1048): error opening trace file: No such file or directory (2) 
03-03 15:22:12.063: D/AndroidRuntime(1048): Shutting down VM 
03-03 15:22:12.083: W/dalvikvm(1048): threadid=1: thread exiting with uncaught exception (group=0x40a71930) 
03-03 15:22:12.093: E/AndroidRuntime(1048): FATAL EXCEPTION: main 
03-03 15:22:12.093: E/AndroidRuntime(1048): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.graphdraw/com.example.graphdraw.MainActivity}: java.lang.NullPointerException 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2106) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.app.ActivityThread.access$600(ActivityThread.java:141) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.os.Handler.dispatchMessage(Handler.java:99) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.os.Looper.loop(Looper.java:137) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.app.ActivityThread.main(ActivityThread.java:5041) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at java.lang.reflect.Method.invokeNative(Native Method) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at java.lang.reflect.Method.invoke(Method.java:511) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at dalvik.system.NativeStart.main(Native Method) 
03-03 15:22:12.093: E/AndroidRuntime(1048): Caused by: java.lang.NullPointerException 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.content.ContextWrapper.getResources(ContextWrapper.java:89) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:78) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.view.View.<init>(View.java:3226) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.view.ViewGroup.<init>(ViewGroup.java:420) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.widget.LinearLayout.<init>(LinearLayout.java:168) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at com.jjoe64.graphview.GraphView.<init>(GraphView.java:258) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at com.jjoe64.graphview.LineGraphView.<init>(LineGraphView.java:20) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at com.example.graphdraw.MainActivity.<init>(MainActivity.java:27) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at java.lang.Class.newInstanceImpl(Native Method) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at java.lang.Class.newInstance(Class.java:1319) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.app.Instrumentation.newActivity(Instrumentation.java:1054) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097) 
03-03 15:22:12.093: E/AndroidRuntime(1048):  ... 11 more 
03-03 15:22:15.763: I/Process(1048): Sending signal. PID: 1048 SIG: 9 

感謝您的幫助。

Answer 1

Context可從onCreate()方法開始Activity。如果將GraphView初始化爲一個字段，則代碼將失敗，因爲Context尚不可用（您應該永遠不要像GraphView那樣初始化Views）。因此，而不是：

GraphView Grafico = new LineGraphView(this, "GraphTest"); 

做到這一點：

GraphView Grafico; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    Grafico = new LineGraphView(this, "GraphTest"); 
} 

此外，你應該直接使用GraphView項目之一或直接在libs文件夾使用jar（見this answer）添加庫。