这是我在android中的条形图代码我得到以下错误[关闭]

Question

这是我的主要活动类

    package android.graph.graphview;

    import android.app.Activity;
    import android.graphics.Canvas;
    import android.os.Bundle;

    public class GraphView extends Activity {
/** Called when the activity is first created. */
GraphViewDemo mGraphViewDemo;
Canvas mCanvas = new Canvas();

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    mGraphViewDemo = (GraphViewDemo) findViewById(R.id.graphview);
    float[] values = new float[] { 1.0f, 6.0f, 7.0f, 8.0f, 9.0f, 3.0f,
            2.0f, 10.0f, 6.0f, 7.0f, 8.0f, 9.0f, 1.0f };
    String[] verlabels = new String[] { "great", "ok", "bad" };
    String[] horlabels = new String[] { "today", "tomorrow", "next week",
            "next month", "Next Year" };
    mGraphViewDemo = new GraphViewDemo(this, values, "GraphViewDemo",
            horlabels, verlabels, GraphViewDemo.BAR);
    mGraphViewDemo.draw(mCanvas);

}

}

这是我在 GraphViewDemo.java 中为绘制条形图定义视图的类

    package android.graph.graphview;

    import android.content.Context;
    import android.graphics.Canvas;
    import android.graphics.Color;
    import android.graphics.Paint;
    import android.graphics.Paint.Align;
    import android.util.AttributeSet;
    import android.util.Log;
    import android.view.View;

    public class GraphViewDemo extends View {

public static boolean BAR = true;
public static boolean LINE = false;

private Paint paint;
private float[] values;
private String[] horlabels;
private String[] verlabels;
private String title;
private boolean type;

public GraphViewDemo(Context context, AttributeSet attrs) {
    super(context, attrs);
}

public GraphViewDemo(Context context, float[] values, String title,
        String[] horlabels, String[] verlabels, boolean type) {
    super(context);
    if (values == null)
        values = new float[0];
    else
        this.values = values;
    if (title == null)
        title = "";
    else
        this.title = title;
    if (horlabels == null)
        this.horlabels = new String[0];
    else
        this.horlabels = horlabels;
    if (verlabels == null)
        this.verlabels = new String[0];
    else
        this.verlabels = verlabels;
    this.type = type;
    paint = new Paint();
}

@Override
protected void onDraw(Canvas canvas) {
    float border = 20;
    float horstart = border * 2;
    float height = getHeight();
    float width = getWidth() - 1;
    float max = getMax();    << also here i get NPE error in my Logcat >>
    float min = getMin();
    float diff = max - min;
    float graphheight = height - (2 * border);
    float graphwidth = width - (2 * border);


    paint.setTextAlign(Align.LEFT);
    int vers = verlabels.length - 1;
    for (int i = 0; i < verlabels.length; i++) {
        // paint.setColor(Color.RED);
        float y = ((graphheight / vers) * i) + border;
        // canvas.drawLine(horstart, y, width, y, paint);
        paint.setColor(Color.WHITE);
        canvas.drawText(verlabels[i], 0, y, paint);
    }
    int hors = horlabels.length - 1;
    for (int i = 0; i < horlabels.length; i++) {
        // paint.setColor(Color.BLUE);
        float x = ((graphwidth / hors) * i) + horstart;
        // canvas.drawLine(x, height - 50, x, border, paint);
        paint.setTextAlign(Align.CENTER);
        if (i == horlabels.length - 1)
            paint.setTextAlign(Align.RIGHT);
        if (i == 0)
            paint.setTextAlign(Align.LEFT);
        paint.setColor(Color.WHITE);
        canvas.drawText(horlabels[i], x, height - 4, paint);
    }

    paint.setTextAlign(Align.CENTER);
    canvas.drawText(title, (graphwidth / 2) + horstart, border - 4, paint);

    if (max != min) {
        paint.setColor(Color.LTGRAY);
        if (type == BAR) {
            int minus = 10;
            float datalength = values.length;
            float colwidth = (width - (2 * border)) / datalength;
            for (int i = 0; i < values.length; i++) {
                float val = values[i] - min;
                float rat = val / diff;
                float h = graphheight * rat;
                canvas.drawRect((i * colwidth) + horstart,
                        ((border - h) + graphheight) + 100,
                        (((i * colwidth) + horstart) + (colwidth - 1))
                                - minus, ((height - (border - 1) - 50)),
                        paint);
                // (border - h)+ graphheight

            }

        }
    }
}

private float getMax() {
    float largest = Integer.MIN_VALUE;
    for (int i = 0; i < values.length; i++)   <<in this line i get NPE
        // if (values[i] > largest)
        // largest = values[i];
        largest = 10;
    Log.v("log_tag", "All Data max of getmax " + largest);
    return largest;
}

private float getMin() {
    float smallest = Integer.MAX_VALUE;
    for (int i = 0; i < values.length; i++)
        // if (values[i] < smallest)
        smallest = -5;
    Log.v("log_tag", "All Data min get min  " + smallest);
    return smallest;
}

}

现在这是我的 main.xml 文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="fill_parent"
android:layout_height="fill_parent">
<TextView android:layout_width="fill_parent"
    android:layout_height="wrap_content" android:text="@string/hello" />
<android.graph.graphview.GraphViewDemo
    android:layout_width="wrap_content" android:layout_height="wrap_content"
    android:id="@+id/graphview" />
</LinearLayout>

这是我的错误，所以请解决这个问题。

    05-16 16:43:17.392: ERROR/AndroidRuntime(822): FATAL EXCEPTION: main
    05-16 16:43:17.392: ERROR/AndroidRuntime(822): java.lang.NullPointerException
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at            android.graph.graphview.GraphViewDemo.getMax(GraphViewDemo.java:159)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.graph.graphview.GraphViewDemo.onDraw(GraphViewDemo.java:57)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.View.draw(View.java:6740)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.ViewGroup.drawChild(ViewGroup.java:1640)      
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1367)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.ViewGroup.drawChild(ViewGroup.java:1638)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1367) 
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.View.draw(View.java:6743)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.widget.FrameLayout.draw(FrameLayout.java:352)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.ViewGroup.drawChild(ViewGroup.java:1640 )
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1367)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.ViewGroup.drawChild(ViewGroup.java:1638)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1367)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.View.draw(View.java:6743)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.widget.FrameLayout.draw(FrameLayout.java:352)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at com.android.internal.policy.impl.PhoneWindow$DecorView.draw(PhoneWindow.java:1842)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.ViewRoot.draw(ViewRoot.java:1407)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.ViewRoot.performTraversals(ViewRoot.java:1163)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1727)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.os.Handler.dispatchMessage(Handler.java:99)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.os.Looper.loop(Looper.java:123)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at android.app.ActivityThread.main(ActivityThread.java:4627)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at java.lang.reflect.Method.invokeNative(Native Method)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at java.lang.reflect.Method.invoke(Method.java:521)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
    05-16 16:43:17.392: ERROR/AndroidRuntime(822):     at dalvik.system.NativeStart.main(Native Method)

我希望这个解决方案在 xml 文件中而不是任何内容视图，所以请检查它。

Answer 1

mGraphViewDemo = new GraphViewDemo(this, values, "GraphViewDemo",
        horlabels, verlabels, GraphViewDemo.BAR);
mGraphViewDemo.draw(mCanvas);

我认为，您的意图是覆盖已经膨胀的视图对象。这不会执行此操作，因此您仍然拥有未正确初始化并因此得到 NullPointerException 的视图。从 xml 中删除视图，然后将创建的（在 onCreate() 中）视图添加到布局中。

setContentView(R.layout.main);
LinearLayout layout = (LinearLayout) findViewById(R.id.layout);// don't forget to mention id in xml
float[] values = new float[] { 1.0f, 6.0f, 7.0f, 8.0f, 9.0f, 3.0f,
        2.0f, 10.0f, 6.0f, 7.0f, 8.0f, 9.0f, 1.0f };
String[] verlabels = new String[] { "great", "ok", "bad" };
String[] horlabels = new String[] { "today", "tomorrow", "next week",
        "next month", "Next Year" };
mGraphViewDemo = new GraphViewDemo(this, values, "GraphViewDemo",
        horlabels, verlabels, GraphViewDemo.BAR);
layout.add(mGraphViewDemo);

此外，您不应实例化 Canvas 对象，也不应手动尝试绘制视图。所有自定义绘图都必须放在视图的 onDraw() 方法中。

Answer 2

getMax 中 NPE 的唯一原因是 values 等于 null。

这是在构造函数中在此代码中启动的：

public GraphViewDemo(Context context, float[] values, String title,
    String[] horlabels, String[] verlabels, boolean type) {
  super(context);

  if (values == null)
    values = new float[0];
  else
    this.values = values;

看看如果你创建一个实例并将null 传递给values 会发生什么？ local 变量values 被初始化为一个大小为零的数组，而class 属性 values 仍然是null。

这将解决问题：

  if (values == null)
    this.values = new float[0];  // <-- change in this line
  else
    this.values = values;

（顺便说一句 - 一个非常常见的错误......）

---

View 的 Javadoc 声明：

View(Context context, AttributeSet attrs)
    Constructor that is called when inflating a view from XML.

很有可能，您的实际View 子类实例是使用另一个已定义且不初始化字段的构造函数创建的。只需在其他构造函数主体中添加一条日志语句即可检查是否是这种情况。