Android：無法連接兩個數組

Question

我想將兩個String數組的數據串聯成第三，但每次使用此代碼來串聯數組時，我的應用程序都會停止工作並關閉：

ArrayList<String> SnoTitles3;
SnoTitles3 = new ArrayList<String>();
SnoTitles3.addAll(Arrays.asList(SnoTitles2));
SnoTitles3.addAll(Arrays.asList(SnoTitles1));
String [] concatedArgs = SnoTitles3.toArray(
                             new String[SnoTitles1.length +SnoTitles2.length]);

我已經讀過一個Stackoverflow問題，需要將數組SnoTitles1和SnoTitles2聲明為final才能將它們連接成第三個數組，但是我不能在這兩個數組中使用final關鍵字，因為最初這些數組的大小是不確定的但是這些數組中的大小和數據是在串聯之前聲明的。

這兩個數組的數據和大小是從XML獲取的，但是所有操作都在連接部分之前完成。

還有一件事，當我刪除以上代碼行/

Logcat輸出：

09-29 18:16:55.327: E/SocketStream(333): readFully was waiting for 403440 bytes, got 49152 
09-29 18:16:55.327: E/SocketStream(333): readFully was waiting for 354288 bytes, got 49152 
09-29 18:16:55.331: E/SocketStream(333): readFully was waiting for 305136 bytes, got 49152 
09-29 18:16:55.331: E/SocketStream(333): readFully was waiting for 255984 bytes, got 16384 
09-29 18:16:55.331: E/SocketStream(333): readFully was waiting for 239600 bytes, got 49152 
09-29 18:16:55.331: E/SocketStream(333): readFully was waiting for 190448 bytes, got 49152 
09-29 18:16:55.331: E/SocketStream(333): readFully was waiting for 141296 bytes, got 49152 
09-29 18:16:55.331: E/SocketStream(333): readFully was waiting for 92144 bytes, got 5524 
09-29 18:16:55.331: E/SocketStream(333): readFully was waiting for 86620 bytes, got 49152 
09-29 18:16:55.331: E/SocketStream(333): readFully was waiting for 37468 bytes, got 32768 
09-29 18:16:55.367: E/Trace(1028): error opening trace file: No such file or directory (2)
09-29 18:16:55.383: E/jdwp(1028): Failed writing handshake bytes: Broken pipe (-1 of 14)
09-29 18:16:55.447: E/AndroidRuntime(1028): FATAL EXCEPTION: main
09-29 18:16:55.447: E/AndroidRuntime(1028): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.news.securitynews/com.news.securitynews.MainActivity}: java.lang.NullPointerException
09-29 18:16:55.447: E/AndroidRuntime(1028):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at android.app.ActivityThread.access$600(ActivityThread.java:130)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at android.os.Looper.loop(Looper.java:137)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at android.app.ActivityThread.main(ActivityThread.java:4745)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at java.lang.reflect.Method.invokeNative(Native Method)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at java.lang.reflect.Method.invoke(Method.java:511)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at dalvik.system.NativeStart.main(Native Method)
09-29 18:16:55.447: E/AndroidRuntime(1028): Caused by: java.lang.NullPointerException
09-29 18:16:55.447: E/AndroidRuntime(1028):     at java.util.Arrays$ArrayList.<init>(Arrays.java:38)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at java.util.Arrays.asList(Arrays.java:154)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at com.news.securitynews.MainActivity.onCreate(MainActivity.java:61)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at android.app.Activity.performCreate(Activity.java:5008)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
09-29 18:16:55.447: E/AndroidRuntime(1028):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
09-29 18:16:55.447: E/AndroidRuntime(1028):     ... 11 more
09-29 18:16:55.467: E/SocketStream(333): readFully was waiting for 403440 bytes, got 49152 
09-29 18:16:55.467: E/SocketStream(333): readFully was waiting for 354288 bytes, got 49152 
09-29 18:16:55.467: E/SocketStream(333): readFully was waiting for 305136 bytes, got 49152 
09-29 18:16:55.467: E/SocketStream(333): readFully was waiting for 255984 bytes, got 49152 
09-29 18:16:55.467: E/SocketStream(333): readFully was waiting for 206832 bytes, got 196608 

更新：

我已經意識到我的程序沒有什么問題，數組SnoTitles1和SnoTitles2在類的頂部被初始化，但是數據是在公共方法中添加到其中的，這就是我的意思，

這是MainActivity類，

public class MainActivity extends ListActivity {

    public String[] SnoTitles1;
    public String[] SnoTitles2;
    ArrayList<String> SnoTitles3;
    String myString = "";


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


/* here getBlogPostsTask.execute(); and getBlogPostsTask.execute1(); add the data inside the arrays using the method update_data1() and update_data2(), which are defined at the bottom.*/


            GetJSONTitles getBlogPostsTask = new GetJSONTitles();
            getBlogPostsTask.execute();

            GetXMLTitles getBlogPostsTask1 = new GetXMLTitles();
            getBlogPostsTask1.execute(); 

/ *執行上述代碼時，將更新數組SnoTitles1和SnoTitles2的值，因此我嘗試在下面將它們連接起來，* /

SnoTitles3.addAll(Arrays.asList(SnoTitles2));
SnoTitles3.addAll(Arrays.asList(SnoTitles1));
String [] concatedArgs = SnoTitles3.toArray(new String[SnoTitles1.length+SnoTitles2.length]);

這是方法update_data1和update_data2，它們修改了數組的值，

    public void update_data2(){
        SnoTitles2 = new String[fetchedXMLTitles.getTitle().size()];
        for (int i = 0; i < fetchedXMLTitles.getTitle().size(); i++) {      
            SnoTitles2[i] = fetchedXMLTitles.getTitle().get(i); 
        }
    }


        public void update_data1(){
        SnoTitles1 = new String[fetchedXMLTitles.getTitle().size()];
        for (int i = 0; i < fetchedXMLTitles.getTitle().size(); i++) {      
            SnoTitles2[i] = fetchedXMLTitles.getTitle().get(i); 
        }
    }


    }

這兩種方法都在同一類中。 我沒有顯示整個代碼，因為它太復雜了，在這里很難解釋。

關鍵是，如果在方法內部修改了數組SnoTitles1和SnoTitles2，那么為什么不能將它們串聯起來呢？

Answer 1

-使用System.arraycopy可以提供最佳性能。

public static String[] concatArrays(String[] src1, String[] src2) {
    if (src1 == null) {
        throw new IllegalArgumentException("src1 is required.")
    }        
    if (src2 == null) {
        throw new IllegalArgumentException("src2 is required.")
    }        

    String[] result = new String[src1.length + src2.length];

    System.arraycopy(src1, 0, result, 0, src1.length);
    System.arraycopy(src2, 0, result, src1.length, src2.length);

    return result;
}

-回到您的問題：

如果在所有update_data1和update_data2完成之前執行串聯兩個數組，則將收到NullPointerException 。 這意味着在執行數組串聯之前，必須確保這兩個方法稱為。 由於這兩個方法在不同的線程中調用（我猜），所以最好的方法是您有一個COUNTER。 請查看我的解決方案：

// Total pending load is 2
AtomicInteger counter = new AtomicInteger(2);


public void update_data2(){
    SnoTitles2 = new String[fetchedXMLTitles.getTitle().size()];
    for (int i = 0; i < fetchedXMLTitles.getTitle().size(); i++) {      
        SnoTitles2[i] = fetchedXMLTitles.getTitle().get(i); 
    }

    if (counter.decrementAndGet() == 0) {

         // Perform array concatenation here

         concatedArgs = concateString(SnoTitles1, SnoTitles2);
    }
}


public void update_data1(){
    SnoTitles1 = new String[fetchedXMLTitles.getTitle().size()];
    for (int i = 0; i < fetchedXMLTitles.getTitle().size(); i++) {      
        SnoTitles1[i] = fetchedXMLTitles.getTitle().get(i); 
    }

    if (counter.decrementAndGet() == 0) {

         // Perform array concatenation here

         concatedArgs = concateString(SnoTitles1, SnoTitles2);
    }
}