Android Studio 开发 - 创建一个按钮来启动 activity
Android Studio Development - Creating a button to launch an activity
我是 Android 开发的新手 我需要一个按钮来调用 activity，同时让我的主要 activity 在按下后退按钮时可用。我已经用谷歌搜索了几天，但找不到可行的解决方案。当我在我的设备 (Samsung Galaxy S4) 上调试应用程序时，我收到一条错误消息，指出 "Unfortunately [App Name] has stopped."
这是我的清单：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.poweredbyite.phoenixcompanion" >
<application
android:allowBackup="true"
android:icon="@drawable/icon"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name=".Splash"
android:label="@string/title_activity_splash" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".MainActivity"
android:label="@string/app_name" >
</activity>
<activity
android:name=".ResistanceCalculator"
android:label="@string/title_activity_resistance_calculator" >
</activity>
</application>
还有我的MainActivity.java
public class MainActivity extends ActionBarActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button btnResCalc = (Button) findViewById(R.id.buttonResistanceCalculator);
btnResCalc.setOnClickListener(new Button.OnClickListener() {
@Override
public void onClick(View v) {
startActivity(new Intent(MainActivity.this, ResistanceCalculator.class));
}
});
}
@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);
}
}
还有 Activity 我想打电话给：
public class ResistanceCalculator extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final Button go = (Button)findViewById(R.id.buttonGo);
go.setOnClickListener(new Button.OnClickListener() {
@Override
public void onClick(View view) {
// Create references to fields
final EditText inputMinCount = (EditText) findViewById(R.id.inputMinCount);
final EditText inputMaxCount = (EditText) findViewById(R.id.inputMaxCount);
final EditText inputMinOhms = (EditText) findViewById(R.id.inputMinOhms);
final EditText inputMaxOhms = (EditText) findViewById(R.id.inputMaxOhms);
final EditText inputResistance = (EditText) findViewById(R.id.inputResistance);
inputMinCount.addTextChangedListener(new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence charSequence, int i, int i2, int i3) {
}
@Override
public void onTextChanged(CharSequence charSequence, int i, int i2, int i3) {
go.setEnabled(!inputMinCount.getText().toString().trim().isEmpty() &&
!inputMaxCount.getText().toString().trim().isEmpty() &&
!inputMinOhms.getText().toString().trim().isEmpty() &&
!inputMaxOhms.getText().toString().trim().isEmpty() &&
!inputResistance.getText().toString().trim().isEmpty());
}
@Override
public void afterTextChanged(Editable editable) {
}
});
// get values from input
String sMinCount = inputMinCount.getText().toString();
String sMaxCount = inputMaxCount.getText().toString();
String sMinOhms = inputMinOhms.getText().toString();
String sMaxOhms = inputMaxOhms.getText().toString();
String sResistance = inputResistance.getText().toString();
//validate
String[] values = {sMaxCount, sMinCount, sMaxOhms, sMinOhms, sResistance};
for (int i = 0; i < values.length - 1; i++) {
if (values[i].matches("")) {
toast("Values cannot be left empty");
return;
}
}
// Grab values from editText fields
int min_c = Integer.parseInt(sMinCount);
int max_c = Integer.parseInt(sMaxCount);
float min_ohms = Float.parseFloat(sMinOhms);
float max_ohms = Float.parseFloat(sMaxOhms);
float med_ohms = (min_ohms + max_ohms) / 2;
float resistance = Float.parseFloat(sResistance);
if (isOdd(max_c) || isOdd(min_c)) {
toast("Counts must be even numbers");
return;
}
if (min_c > max_c) {
toast("Maximum count must be larger than minimum count");
return;
}
if (min_ohms > max_ohms) {
toast("Maximum resistance must be larger than minimum resistance");
return;
}
if (resistance > min_ohms && resistance < max_ohms) {
toast("Part already in range");
return;
}
int count = (int) Math.floor(med_ohms / (resistance / min_c));
//toast(Integer.toString(count));
for(int i=1; i<11; i++){
System.out.println("Count is: " + i);
}
if (count < min_c || count > max_c) {
toast("Input out of Range: Projected count: " + count);
return;
}
if (isOdd(count)) {
count--;
}
/*AlertDialog alertDialog = new AlertDialog.Builder(main.this).create();
alertDialog.setTitle("Final Count");
alertDialog.setMessage(Integer.toString(count));
alertDialog.show();*/
return;
}
});
}
@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.
switch (item.getItemId()) {
}
return super.onOptionsItemSelected(item);
}
boolean isOdd( int val ) { return (val & 0x01) != 0; }
void toast(String msg) {
Context context = getApplicationContext();
int duration = Toast.LENGTH_SHORT;
Toast.makeText(context, msg, duration).show();
}
void setFields(String part) {
EditText inputMinCount = (EditText)findViewById(R.id.inputMinCount);
EditText inputMaxCount = (EditText)findViewById(R.id.inputMaxCount);
EditText inputMinOhms = (EditText)findViewById(R.id.inputMinOhms);
EditText inputMaxOhms = (EditText)findViewById(R.id.inputMaxOhms);
// set values
inputMinCount.setText(Integer.toString(72));
inputMaxCount.setText(Integer.toString(78));
inputMinOhms.setText(Float.toString((float) 0.364));
inputMaxOhms.setText(Float.toString((float) 0.382));
}
}
这是日志错误
01-29 02:21:16.016 30867-30867/com.poweredbyite.phoenixcompanion E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.poweredbyite.phoenixcompanion, PID: 30867
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.poweredbyite.phoenixcompanion/com.poweredbyite.phoenixcompanion.ResistanceCalculator}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305)
at
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2363)
at android.app.ActivityThread.access0(ActivityThread.java:161)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1265)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:157)
at android.app.ActivityThread.main(ActivityThread.java:5356)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.poweredbyite.phoenixcompanion.ResistanceCalculator.onCreate(ResistanceCalculator.java:28)
at android.app.Activity.performCreate(Activity.java:5426)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2269)
at
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2363)
at android.app.ActivityThread.access0(ActivityThread.java:161)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1265)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:157)
at android.app.ActivityThread.main(ActivityThread.java:5356)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
at dalvik.system.NativeStart.main(Native Method)
01-29 02:21:48.508 30867-30867/com.poweredbyite.phoenixcompanion I/Process﹕ Sending signal. PID: 30867 SIG: 9
如有任何帮助，我们将不胜感激。
您的两个活动在 setContentView 中具有相同的布局。 （错误原因）
在清单中第二个 activity 的 activity 部分设置属性 android:parentActivityName=".MainActivity"。它将在您的第二个 activity.
按下后退按钮时打开您的主要 activity
我是 Android 开发的新手 我需要一个按钮来调用 activity，同时让我的主要 activity 在按下后退按钮时可用。我已经用谷歌搜索了几天，但找不到可行的解决方案。当我在我的设备 (Samsung Galaxy S4) 上调试应用程序时，我收到一条错误消息，指出 "Unfortunately [App Name] has stopped."
这是我的清单：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.poweredbyite.phoenixcompanion" >
<application
android:allowBackup="true"
android:icon="@drawable/icon"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name=".Splash"
android:label="@string/title_activity_splash" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".MainActivity"
android:label="@string/app_name" >
</activity>
<activity
android:name=".ResistanceCalculator"
android:label="@string/title_activity_resistance_calculator" >
</activity>
</application>
还有我的MainActivity.java
public class MainActivity extends ActionBarActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button btnResCalc = (Button) findViewById(R.id.buttonResistanceCalculator);
btnResCalc.setOnClickListener(new Button.OnClickListener() {
@Override
public void onClick(View v) {
startActivity(new Intent(MainActivity.this, ResistanceCalculator.class));
}
});
}
@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);
}
}
还有 Activity 我想打电话给：
public class ResistanceCalculator extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final Button go = (Button)findViewById(R.id.buttonGo);
go.setOnClickListener(new Button.OnClickListener() {
@Override
public void onClick(View view) {
// Create references to fields
final EditText inputMinCount = (EditText) findViewById(R.id.inputMinCount);
final EditText inputMaxCount = (EditText) findViewById(R.id.inputMaxCount);
final EditText inputMinOhms = (EditText) findViewById(R.id.inputMinOhms);
final EditText inputMaxOhms = (EditText) findViewById(R.id.inputMaxOhms);
final EditText inputResistance = (EditText) findViewById(R.id.inputResistance);
inputMinCount.addTextChangedListener(new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence charSequence, int i, int i2, int i3) {
}
@Override
public void onTextChanged(CharSequence charSequence, int i, int i2, int i3) {
go.setEnabled(!inputMinCount.getText().toString().trim().isEmpty() &&
!inputMaxCount.getText().toString().trim().isEmpty() &&
!inputMinOhms.getText().toString().trim().isEmpty() &&
!inputMaxOhms.getText().toString().trim().isEmpty() &&
!inputResistance.getText().toString().trim().isEmpty());
}
@Override
public void afterTextChanged(Editable editable) {
}
});
// get values from input
String sMinCount = inputMinCount.getText().toString();
String sMaxCount = inputMaxCount.getText().toString();
String sMinOhms = inputMinOhms.getText().toString();
String sMaxOhms = inputMaxOhms.getText().toString();
String sResistance = inputResistance.getText().toString();
//validate
String[] values = {sMaxCount, sMinCount, sMaxOhms, sMinOhms, sResistance};
for (int i = 0; i < values.length - 1; i++) {
if (values[i].matches("")) {
toast("Values cannot be left empty");
return;
}
}
// Grab values from editText fields
int min_c = Integer.parseInt(sMinCount);
int max_c = Integer.parseInt(sMaxCount);
float min_ohms = Float.parseFloat(sMinOhms);
float max_ohms = Float.parseFloat(sMaxOhms);
float med_ohms = (min_ohms + max_ohms) / 2;
float resistance = Float.parseFloat(sResistance);
if (isOdd(max_c) || isOdd(min_c)) {
toast("Counts must be even numbers");
return;
}
if (min_c > max_c) {
toast("Maximum count must be larger than minimum count");
return;
}
if (min_ohms > max_ohms) {
toast("Maximum resistance must be larger than minimum resistance");
return;
}
if (resistance > min_ohms && resistance < max_ohms) {
toast("Part already in range");
return;
}
int count = (int) Math.floor(med_ohms / (resistance / min_c));
//toast(Integer.toString(count));
for(int i=1; i<11; i++){
System.out.println("Count is: " + i);
}
if (count < min_c || count > max_c) {
toast("Input out of Range: Projected count: " + count);
return;
}
if (isOdd(count)) {
count--;
}
/*AlertDialog alertDialog = new AlertDialog.Builder(main.this).create();
alertDialog.setTitle("Final Count");
alertDialog.setMessage(Integer.toString(count));
alertDialog.show();*/
return;
}
});
}
@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.
switch (item.getItemId()) {
}
return super.onOptionsItemSelected(item);
}
boolean isOdd( int val ) { return (val & 0x01) != 0; }
void toast(String msg) {
Context context = getApplicationContext();
int duration = Toast.LENGTH_SHORT;
Toast.makeText(context, msg, duration).show();
}
void setFields(String part) {
EditText inputMinCount = (EditText)findViewById(R.id.inputMinCount);
EditText inputMaxCount = (EditText)findViewById(R.id.inputMaxCount);
EditText inputMinOhms = (EditText)findViewById(R.id.inputMinOhms);
EditText inputMaxOhms = (EditText)findViewById(R.id.inputMaxOhms);
// set values
inputMinCount.setText(Integer.toString(72));
inputMaxCount.setText(Integer.toString(78));
inputMinOhms.setText(Float.toString((float) 0.364));
inputMaxOhms.setText(Float.toString((float) 0.382));
}
}
这是日志错误
01-29 02:21:16.016 30867-30867/com.poweredbyite.phoenixcompanion E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.poweredbyite.phoenixcompanion, PID: 30867
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.poweredbyite.phoenixcompanion/com.poweredbyite.phoenixcompanion.ResistanceCalculator}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305)
at
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2363)
at android.app.ActivityThread.access0(ActivityThread.java:161)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1265)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:157)
at android.app.ActivityThread.main(ActivityThread.java:5356)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.poweredbyite.phoenixcompanion.ResistanceCalculator.onCreate(ResistanceCalculator.java:28)
at android.app.Activity.performCreate(Activity.java:5426)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2269)
at
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2363)
at android.app.ActivityThread.access0(ActivityThread.java:161)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1265)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:157)
at android.app.ActivityThread.main(ActivityThread.java:5356)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
at dalvik.system.NativeStart.main(Native Method)
01-29 02:21:48.508 30867-30867/com.poweredbyite.phoenixcompanion I/Process﹕ Sending signal. PID: 30867 SIG: 9
如有任何帮助，我们将不胜感激。
您的两个活动在 setContentView 中具有相同的布局。 （错误原因）
在清单中第二个 activity 的 activity 部分设置属性 android:parentActivityName=".MainActivity"。它将在您的第二个 activity.
按下后退按钮时打开您的主要 activity