java - 为什么java认为这个方法是被声明的，而不是被调用的？

Question

我是编程新手，我已经在 Android 上花了一些时间了。我正在尝试制作一个简单的应用程序(只有一个简单的类(class))来计算你需要在期末考试中获得什么才能在类(class)中获得一定的成绩。在第 5 行，我分配了一个按钮来计算，我收到一条错误消息“该行有多个标记

- Return type for the method is missing
- Syntax error, insert ";" to complete 
 FieldDeclaration
- Syntax error on token ".", ... expected
- Syntax error on token ")", { expected after this 
 token"

我能够成功编译一些极其相似的东西，但我不记得我改变了什么使它不起作用!这是我的简单类(class):非常感谢任何帮助。

public class MainActivity extends Activity implements View.OnClickListener  {

Button calculate;
EditText grade, wantedGrade, finalValue;
calculate = (Button) findViewById(R.id.button1);

grade = (EditText) findViewById(R.id.editText1);
wantedGrade = (EditText) findViewById(R.id.editText2);
finalValue = (EditText) findViewById(R.id.editText3);

@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;
}

/** calculates the grade and alerts the result */
public void reCalc(View view)
{

    // calculate the grade
    // (TotalGrade - (currentGrade * .8)) / .2) = finalGrade
    // where .8 and .2 are the values of the normal semester 
    // weight and final weight, respectively
    double target;
    double current = Double.parseDouble(grade.getText().toString());
    double desired = Double.parseDouble(wantedGrade.getText().toString());
    double finalWeight = Double.parseDouble(finalValue.getText().toString()) / 100;
    double normalWeight = 1.0 - finalWeight;


    target = (desired - (current * normalWeight)) / finalWeight; 

    AlertDialog gradeMessenger = new AlertDialog.Builder(this).create();     
    gradeMessenger.setMessage("You need a " + target + " percent on the final to get a " + desired + " in the class.");
    gradeMessenger.setCancelable(true);
}

@Override
public void onClick(View v) {
    // TODO Auto-generated method stub

}}

编辑:我目前没有足够的声誉来支持大家投票，但现在效果很好，感谢您的帮助。我非常感激。

Answer 1

更改为

@Override
protected void onCreate(Bundle savedInstanceState) 
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main); 
    // initialize after setContentview
    calculate = (Button) findViewById(R.id.button1);

    grade = (EditText) findViewById(R.id.editText1);
    wantedGrade = (EditText) findViewById(R.id.editText2);
    finalValue = (EditText) findViewById(R.id.editText3);

}

findViewById 在当前扩展布局中查找具有 id 的 View 。所以你需要在setContentView之后初始化 View 。

我猜你也有

android:onClick="reCalc" in xml for button

因此您的 Activity 不需要实现 OnClickListener 并删除

@Override
public void onClick(View v) {
// TODO Auto-generated method stub

}