1.例如下面的代码片段，Toast类的第一个参数接受一个Context对象：
@Override
protected Dialog onCreateDialog(int id) {
switch (id) {
case 0:
Builder builder = new AlertDialog.Builder(this);
builder.setIcon(R.drawable.ic_launcher);
builder.setTitle("This is a dialog with some simple text...");
builder.setPositiveButton("OK",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog,
int whichButton) {
Toast.makeText(getBaseContext(), "OK clicked!",
Toast.LENGTH_SHORT).show();
}
});
builder.setNegativeButton("Cancel",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog,
int whichButton) {
Toast.makeText(getBaseContext(), "Cancel clicked!",
Toast.LENGTH_SHORT).show();
}
});
builder.setMultiChoiceItems(items, itemsChecked,
new DialogInterface.OnMultiChoiceClickListener() {
public void onClick(DialogInterface dialog, int which,
boolean isChecked) {
Toast.makeText(
getBaseContext(),
items[which]
+ (isChecked ? " checked!"
: " unchecked!"),
Toast.LENGTH_SHORT).show();
}
});
return builder.create();
}
return null;
}
@Override
protected Dialog onCreateDialog(int id) {
switch (id) {
case 0:
Builder builder = new AlertDialog.Builder(this);
builder.setIcon(R.drawable.ic_launcher);
builder.setTitle("This is a dialog with some simple text...");
builder.setPositiveButton("OK",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog,
int whichButton) {
Toast.makeText(getBaseContext(), "OK clicked!",
Toast.LENGTH_SHORT).show();
}
});
builder.setNegativeButton("Cancel",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog,
int whichButton) {
Toast.makeText(getBaseContext(), "Cancel clicked!",
Toast.LENGTH_SHORT).show();
}
});
builder.setMultiChoiceItems(items, itemsChecked,
new DialogInterface.OnMultiChoiceClickListener() {
public void onClick(DialogInterface dialog, int which,
boolean isChecked) {
Toast.makeText(
getBaseContext(),
items[which]
+ (isChecked ? " checked!"
: " unchecked!"),
Toast.LENGTH_SHORT).show();
}
});
return builder.create();
}
return null;
}
然而，Toast类并没有直接用在Activity中，它被用在了AlertDialog类中。所以，这里需要通过getBaseContext()方法获取一个Context类的实例。
2.在Activity中动态地创建一个视图的时候也会遇见Context。
例如，如果想通过硬编码动态地创建一个TextView：
TextView tv = new TextView(this); TextView tv = new TextView(this);
TextView的构造器接受一个Context对象，因为Activity类是Context类的子类，所以可以用this关键字来代替这个Conext对象。
提示：
使用this动态地创建视图，例如TextView、Button，存在一个潜在的风险——内存泄漏。所以，尽可能地使用getApplicationContext()方法替代this。
