我正在尝试找出在我的应用中抛出NullPointerException的内容，但是，2小时后我仍然什么都没有。 有人能帮助我吗？
主要代码：
public class Main extends Activity implements OnClickListener {
TextView tv_screen;
static final String digits = "0123456789.";
Boolean userIsInTheMiddleOfTypingANumber = false;
Solver solver;
DecimalFormat df = new DecimalFormat("@###########");
@Override
protected void onCreate(Bundle savedInstanceState) {
/* REMOVE TITLE BAR */
this.requestWindowFeature(Window.FEATURE_NO_TITLE);
/* REMOVE NOTIFICATION BAR (AKA FULL SCREEN) */
this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
df.setMinimumFractionDigits(0);
df.setMinimumIntegerDigits(1);
df.setMaximumIntegerDigits(8);
tv_screen = (TextView) findViewById(R.id.tv_screen);
findViewById(R.id.button_1).setOnClickListener(this);
findViewById(R.id.button_2).setOnClickListener(this);
findViewById(R.id.button_3).setOnClickListener(this);
findViewById(R.id.button_4).setOnClickListener(this);
findViewById(R.id.button_5).setOnClickListener(this);
findViewById(R.id.button_6).setOnClickListener(this);
findViewById(R.id.button_7).setOnClickListener(this);
findViewById(R.id.button_8).setOnClickListener(this);
findViewById(R.id.button_9).setOnClickListener(this);
findViewById(R.id.button_0).setOnClickListener(this);
findViewById(R.id.button_multiply).setOnClickListener(this);
findViewById(R.id.button_divide).setOnClickListener(this);
findViewById(R.id.button_minus).setOnClickListener(this);
findViewById(R.id.button_sum).setOnClickListener(this);
}
@Override
public void onClick(View v) {
String buttonPressed = ((Button) v).getText().toString();
if (digits.contains(buttonPressed)) {
// digit was pressed
if (userIsInTheMiddleOfTypingANumber) {
if (buttonPressed.equals(".") && tv_screen.getText().toString().contains(".")) {
// ERROR PREVENTION
// Eliminate entering multiple decimals
} else {
tv_screen.append(buttonPressed);
}
} else {
if (buttonPressed.equals(".")) {
// ERROR PREVENTION
// This will avoid error if only the decimal is hit before an operator, by placing a leading zero
// before the decimal
tv_screen.setText(0 + buttonPressed);
} else {
tv_screen.setText(buttonPressed);
}
userIsInTheMiddleOfTypingANumber = true;
}
} else {
// operation was pressed
if (userIsInTheMiddleOfTypingANumber) {
//Log.w("TAG", "thickness round:" + yyy);
solver.setOperand(Double.parseDouble(tv_screen.getText().toString()));
userIsInTheMiddleOfTypingANumber = false;
}
solver.performOperation(buttonPressed);
tv_screen.setText(df.format(solver.getResult()));
}
}
}
解算器代码：
public class Solver {
// 3 + 6 = 9
// 3 & 6 are called the operand.
// The + is called the operator.
// 9 is the result of the operation.
private double mOperand;
private double mWaitingOperand;
private String mWaitingOperator;
private double mCalculatorMemory;
// operator types
public static final String ADD = "+";
public static final String SUBTRACT = "-";
public static final String MULTIPLY = "x";
public static final String DIVIDE = "/";
public static final String CLEAR = "C" ;
public static final String CLEARMEMORY = "MC";
public static final String ADDTOMEMORY = "M+";
public static final String SUBTRACTFROMMEMORY = "M-";
public static final String RECALLMEMORY = "MR";
public static final String SQUAREROOT = "√";
public static final String SQUARED = "x²";
public static final String INVERT = "1/x";
public static final String TOGGLESIGN = "+/-";
public static final String SINE = "sin";
public static final String COSINE = "cos";
public static final String TANGENT = "tan";
// public static final String EQUALS = "=";
// constructor
public Solver() {
// initialize variables upon start
mOperand = 0;
mWaitingOperand = 0;
mWaitingOperator = "";
mCalculatorMemory = 0;
}
public void setOperand(double operand) {
mOperand = operand;
}
public double getResult() {
return mOperand;
}
// used on screen orientation change
public void setMemory(double calculatorMemory) {
mCalculatorMemory = calculatorMemory;
}
// used on screen orientation change
public double getMemory() {
return mCalculatorMemory;
}
public String toString() {
return Double.toString(mOperand);
}
protected double performOperation(String operator) {
/*
* If you are using Java 7, then you can use switch in place of if statements
*
* switch (operator) {
* case CLEARMEMORY:
* calculatorMemory = 0;
* break;
* case ADDTOMEMORY:
* calculatorMemory = calculatorMemory + operand;
* break;
* etc...
* }
*/
if (operator.equals(CLEAR)) {
mOperand = 0;
mWaitingOperator = "";
mWaitingOperand = 0;
// mCalculatorMemory = 0;
} else if (operator.equals(CLEARMEMORY)) {
mCalculatorMemory = 0;
} else if (operator.equals(ADDTOMEMORY)) {
mCalculatorMemory = mCalculatorMemory + mOperand;
} else if (operator.equals(SUBTRACTFROMMEMORY)) {
mCalculatorMemory = mCalculatorMemory - mOperand;
} else if (operator.equals(RECALLMEMORY)) {
mOperand = mCalculatorMemory;
} else if (operator.equals(SQUAREROOT)) {
mOperand = Math.sqrt(mOperand);
} else if (operator.equals(SQUARED)) {
mOperand = mOperand * mOperand;
} else if (operator.equals(INVERT)) {
if (mOperand != 0) {
mOperand = 1 / mOperand;
}
} else if (operator.equals(TOGGLESIGN)) {
mOperand = -mOperand;
} else if (operator.equals(SINE)) {
mOperand = Math.sin(Math.toRadians(mOperand)); // Math.toRadians(mOperand) converts result to degrees
} else if (operator.equals(COSINE)) {
mOperand = Math.cos(Math.toRadians(mOperand)); // Math.toRadians(mOperand) converts result to degrees
} else if (operator.equals(TANGENT)) {
mOperand = Math.tan(Math.toRadians(mOperand)); // Math.toRadians(mOperand) converts result to degrees
} else {
performWaitingOperation();
mWaitingOperator = operator;
mWaitingOperand = mOperand;
}
return mOperand;
}
protected void performWaitingOperation() {
if (mWaitingOperator.equals(ADD)) {
mOperand = mWaitingOperand + mOperand;
} else if (mWaitingOperator.equals(SUBTRACT)) {
mOperand = mWaitingOperand - mOperand;
} else if (mWaitingOperator.equals(MULTIPLY)) {
mOperand = mWaitingOperand * mOperand;
} else if (mWaitingOperator.equals(DIVIDE)) {
if (mOperand != 0) {
mOperand = mWaitingOperand / mOperand;
}
}
}
}
错误：
01-03 00:46:02.420: E/AndroidRuntime(2558): FATAL EXCEPTION: main
01-03 00:46:02.420: E/AndroidRuntime(2558): java.lang.NullPointerException
01-03 00:46:02.420: E/AndroidRuntime(2558): at com.pipeflexcalculator.calculator.Main.onClick(Main.java:103)
01-03 00:46:02.420: E/AndroidRuntime(2558): at android.view.View.performClick(View.java:4475)
01-03 00:46:02.420: E/AndroidRuntime(2558): at android.view.View$PerformClick.run(View.java:18786)
01-03 00:46:02.420: E/AndroidRuntime(2558): at android.os.Handler.handleCallback(Handler.java:730)
01-03 00:46:02.420: E/AndroidRuntime(2558): at android.os.Handler.dispatchMessage(Handler.java:92)
01-03 00:46:02.420: E/AndroidRuntime(2558): at android.os.Looper.loop(Looper.java:176)
01-03 00:46:02.420: E/AndroidRuntime(2558): at android.app.ActivityThread.main(ActivityThread.java:5419)
01-03 00:46:02.420: E/AndroidRuntime(2558): at java.lang.reflect.Method.invokeNative(Native Method)
01-03 00:46:02.420: E/AndroidRuntime(2558): at java.lang.reflect.Method.invoke(Method.java:525)
01-03 00:46:02.420: E/AndroidRuntime(2558): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)
01-03 00:46:02.420: E/AndroidRuntime(2558): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)
01-03 00:46:02.420: E/AndroidRuntime(2558): at dalvik.system.NativeStart.main(Native Method)
logcat一直指向以下行：
solver.setOperand(Double.parseDouble(tv_screen.getText().toString()));
我试图将“Double .....”更改为“3.0”，所以我确定它在tv_screen视图中不是问题，但仍然会出错。
有什么想法吗？
答案 0 :(得分：2)
您似乎无法在任何地方实例化solver，因此它在onClick()中为空。之前在某处实例化，例如在onCreate()或您声明它的位置：
Solver solver = new Solver();