android应用上的if语句帮助

Question

iv created an android app, what my problem is that when i press the + = - / * buttons before i input a number my app stops working, is there anyway i can make it that if there is no number input it doesnt stop working?

---

package steven.mcilhone.calculatorcoursework;

import java.math.BigDecimal;


import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;


public class Calculator extends Activity {

EditText firstValue;
EditText secondValue;
TextView result;
Button addbtn, subtractbtn, dividebtn, multiplybtn, equalbtn, clearbtn;
BigDecimal firstNum, secondNum;



String Operator = "";

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_calculator);

    addbtn = (Button) findViewById(R.id.addbtnID);
    subtractbtn = (Button) findViewById(R.id.subtractbtnID);
    dividebtn = (Button) findViewById(R.id.dividebtnID);
    multiplybtn = (Button) findViewById(R.id.multiplybtnID);
    equalbtn = (Button) findViewById(R.id.equalbtnID);
    clearbtn = (Button) findViewById(R.id.clearbtn);
    firstValue = (EditText) findViewById(R.id.edttxt1);
    secondValue = (EditText) findViewById(R.id.edttxt1);


    clearbtn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            EditText edttxt1 = (EditText) findViewById(R.id.edttxt1);
            edttxt1.setText("");
        }
    });



    addbtn.setOnClickListener(new View.OnClickListener() {

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

            Operator = "+";
            addandclear();
        }
    });

    subtractbtn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            Operator = "-";             
            addandclear();
        }
    });

    dividebtn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            Operator = "/";
            addandclear();

        }
    });

    multiplybtn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            Operator = "*";
            addandclear();
        }
    });

    equalbtn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            secondNum = new          BigDecimal(secondValue.getText().toString());
            EditText edttxt1 = (EditText) findViewById(R.id.edttxt1);
            edttxt1.setText("");
            if (Operator == "+"){
                edttxt1.setText(firstNum.add(secondNum).toString());
            }
            else if (Operator == "-"){
                edttxt1.setText(firstNum.subtract(secondNum).toString());
            }
            else if (Operator == "/"){
                edttxt1.setText(firstNum.divide(secondNum).toString());
            }   
            else if (Operator == "*"){
                edttxt1.setText(firstNum.multiply(secondNum).toString());
            }   



        }
    });

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.activity_calculator, menu);
    return true;
}


        public void addandclear(){


            firstNum = new BigDecimal(firstValue.getText().toString());             
            EditText edttxt1 = (EditText) findViewById(R.id.edttxt1);
            edttxt1.setText("");
        }



}

---

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent" >

<EditText
    android:id="@+id/edttxt1"
    android:layout_width="250dp"
    android:layout_height="100dp"
    android:layout_alignParentRight="true"
    android:layout_alignParentTop="true"
    android:background="?android:attr/editTextBackground"
    android:ems="15"
    android:inputType="numberDecimal" 
    android:gravity="right"/>

<Button
    android:id="@+id/addbtnID"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_below="@+id/edttxt1"
    android:layout_marginLeft="20dp"
    android:text="@string/add" />

<Button
    android:id="@+id/dividebtnID"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_below="@+id/edttxt1"
    android:layout_toRightOf="@+id/addbtnID"
    android:text="@string/divide" />

<Button
    android:id="@+id/multiplybtnID"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_below="@+id/edttxt1"
    android:layout_toRightOf="@+id/dividebtnID"
    android:text="@string/multiply" />

<Button
    android:id="@+id/subtractbtnID"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_alignBaseline="@+id/multiplybtnID"
    android:layout_alignBottom="@+id/multiplybtnID"
    android:layout_toRightOf="@+id/multiplybtnID"
    android:text="@string/subtract" />

<Button
    android:id="@+id/zerobtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_alignLeft="@+id/addbtnID"
    android:layout_alignParentBottom="true"
    android:layout_alignRight="@+id/twobtn"
    android:text="@string/zero" />

<Button
    android:id="@+id/onebtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/zerobtn"
    android:layout_alignLeft="@+id/zerobtn"
    android:text="@string/one" />

<Button
    android:id="@+id/twobtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/zerobtn"
    android:layout_toLeftOf="@+id/multiplybtnID"
    android:text="@string/two" />

<Button
    android:id="@+id/threebtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/zerobtn"
    android:layout_toLeftOf="@+id/subtractbtnID"
    android:text="@string/three" />

<Button
    android:id="@+id/fourbtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/onebtn"
    android:layout_toLeftOf="@+id/twobtn"
    android:text="@string/four" />

<Button
    android:id="@+id/fivebtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_alignBaseline="@+id/fourbtn"
    android:layout_alignBottom="@+id/fourbtn"
    android:layout_toLeftOf="@+id/threebtn"
    android:text="@string/five" />

<Button
    android:id="@+id/sixbtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_alignBaseline="@+id/fivebtn"
    android:layout_alignBottom="@+id/fivebtn"
    android:layout_toRightOf="@+id/fivebtn"
    android:text="@string/six" />

<Button
    android:id="@+id/sevenbtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/fourbtn"
    android:layout_toLeftOf="@+id/fivebtn"
    android:text="@string/seven" />

<Button
    android:id="@+id/eightbtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/fivebtn"
    android:layout_toLeftOf="@+id/sixbtn"
    android:text="@string/eight" />

<Button
    android:id="@+id/ninebtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_alignBaseline="@+id/eightbtn"
    android:layout_alignBottom="@+id/eightbtn"
    android:layout_toRightOf="@+id/eightbtn"
    android:text="@string/nine" />

<Button
    android:id="@+id/equalbtnID"
    android:layout_width="70dp"
    android:layout_height="120dp"
    android:layout_alignBottom="@+id/threebtn"
    android:layout_alignTop="@+id/sixbtn"
    android:layout_toRightOf="@+id/threebtn"
    android:text="@string/equal" />

<Button
    android:id="@+id/clearbtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/equalbtnID"
    android:layout_alignLeft="@+id/equalbtnID"
    android:text="@string/clear" />

</RelativeLayout>

Answer 1

 if (Operator == "+"){

should be

 if (Operator.eqauls("+")){

Same applies for other if/else blocks in your code.

It is always better to us .equals() instead of == while comparing Strings (except the case of String literals, String literals may pass == condition).

== checks for reference equality. equals() checks for content equality. This discussion may help you.

EDIT:

As TedHopp commented, your code referring to compile time constants, String comparison may not be issue here, still above answers stands as good practice for String comparison.

Because there is no stacktrace available with question, assuming flow satisfies one of the if/else conditions, one other possibility for error is NumberFormatException .

Maek sure the numbers you are reading from EditText are valid Number before creating BigDecimal(String) instance.

Answer 2

In several places, you are converting the user input to a BigDecimal without doing any error checking. In particular, when there is no input, new BigDecimal(String) will generate a NumberFormatException because "" is not a valid number. From the docs :

The string must contain at least one digit in either the integer or the fraction.

Find every place where you are using the user input and put in some error checking. For instance, in addandclear() :

public void addandclear(){
    String val = firstValue.getText().toString();
    try {
        firstNum = new BigDecimal(val);
        EditText edttxt1 = (EditText) findViewById(R.id.edttxt1);
        edttxt1.setText("");
    } catch (NumberFormatException e) {
        if (val.length() == 0) {
            // blank field
        } else {
            // something wrong with the input
        }
    }
}

Answer 3

To reduce code you could add android:onClick="addandclear" to the buttons in the xml (make addandclear take View v as argument). Then get the operator by v.getText().toString()