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);
}
}
这是我的类，我在GraphViewDemo.java中为绘制条形图定义视图
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xml文件中的此解决方案不是任何内容视图，因此请检查它。
答案 0 :(得分：2)
mGraphViewDemo = new GraphViewDemo(this, values, "GraphViewDemo",
horlabels, verlabels, GraphViewDemo.BAR);
mGraphViewDemo.draw(mCanvas);
我认为，你的意图是覆盖已经膨胀的视图对象。这不会这样做，所以你仍然有视图，它没有正确初始化，因此得到NullPointerException。从xml中删除视图，只需将创建的（在onCreate（））视图中添加到布局中。
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，您不应该实例化Canvas对象，也不应该手动尝试绘制视图。必须将所有自定义绘图放在视图的onDraw（）方法中。
答案 1 :(得分：0)
getMax中NPE的唯一原因是values等于null。
这是在此代码的构造函数中启动的：
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;
看看如果您创建实例并为null传递values会发生什么？使用零大小数组初始化本地变量values，而类属性 values仍为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州的Javadoc：
View(Context context, AttributeSet attrs)
Constructor that is called when inflating a view from XML.
很有可能，您的实际View子类实例已使用定义的其他构造函数创建，并且不会初始化字段。只需将log语句添加到其他构造函数体，以检查是否是这种情况。