Java 使按钮不工作?
我通过“btnCalculate.setOnClickListener(new View.OnClickListener()”得到一个错误 “方法调用'btnCalculate.setOnClickListener(新 View.OnClickListener(){@Override…'可能会产生 “java.lang.NullPointerException” 这意味着什么?我如何修复它Java 使按钮不工作?,java,android,Java,Android,我通过“btnCalculate.setOnClickListener(new View.OnClickListener()”得到一个错误 “方法调用'btnCalculate.setOnClickListener(新 View.OnClickListener(){@Override…'可能会产生 “java.lang.NullPointerException” 这意味着什么?我如何修复它 import android.os.Bundle; import android.support.v7.a
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import org.w3c.dom.Text;
public class MainActivity extends AppCompatActivity
{
TextView totalTextView;
EditText edtPercentage;
EditText edtNumber;
float percentage;
float number;
float total;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
percentage = 0;
number = 0;
total = 0;
Button btnCalculate = (Button) findViewById(R.id.btnCalculate);
btnCalculate.setOnClickListener(new View.OnClickListener()
{
@Override
public void onClick(View view)
{
percentage = Float.parseFloat(edtPercentage.getText().toString());
number = Float.parseFloat(edtNumber.getText().toString());
total = number * percentage/100;
totalTextView.setText(String.valueOf(total));
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item)
{
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings)
{
return true;
}
return super.onOptionsItemSelected(item);
}
}
在布局中找不到btnCalculate时,btnCalculate可能为空-要消除可能添加的警告,请执行以下操作:
assert(btnCalculate!=null)
警告之前,您尚未使用
findReferenceByID()
方法“连接”totalTextView
edtPercentage
和edtNumber
。在onCreate
方法中
.....
setContentView(R.layout.activity_main());
totalTextView = (TextView)findViewByID(R.id.<IdOfTextView>);
edtPercentage = (EditText)findViewByID(R.id.<IdOfEditText>);
edtNumber = (EditText)findViewByID(R.id.<IdOfEditText>);
.....
。。。。。
setContentView(R.layout.activity_main());
totalTextView=(TextView)findViewByID(R.id.);
edtPercentage=(EditText)findViewByID(R.id.);
edtNumber=(EditText)findViewByID(R.id.);
.....
消息非常清楚:
此对象TextView totalTextView
必须在您尝试设置其上的文本之前进行初始化(通常在onCreate
方法中),但您忘记了设置,您的应用程序将在单击按钮后立即崩溃
修好它
这样做:
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
totalTextView = (TextView) findViewById(R.id.toolbar); //init the text view....
....
您没有实例化此活动的textviewpost xml