用getCacheDir()的时候报空指针异常

public class MainActivity extends Activity {
	private String path;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		Context context = this;
		path = context.getCacheDir().getPath();// 1
		path = this.getCacheDir().getPath();// 2
		path = getCacheDir().getPath();// 3
		// 在Activity中1、2、3代码等效
	}
}