How to detect screen pressure on android?

Question

I want to get the pressure of the screen. When ill run the application displaying me a message Unfortunately, Application has stopped. I have another question when ill get the pressure of the screen does its possible to convert in a weigh (grams) with some math or physic ? Here is the code i have:

EDIT: I'm running this application on tablet

package com.realscale.stefanrafa;

    import android.app.Activity;
    import android.os.Bundle;
    import android.view.Menu;
    import android.view.MotionEvent;
    import android.widget.TextView;

    public class MainActivity extends Activity {

        TextView textinfo;
        MotionEvent pressure;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            textinfo = (TextView) findViewById(R.id.Information);

            float press = pressure.getPressure();
            textinfo.setText("Scale: " + (int) press);
        }

        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.main, menu);
            return true;
        }

    }

Error logs :

08-30 19:53:40.620: W/ActivityThread(29458): Application com.realscale.stefanrafa can be debugged on port 8100... 08-30 19:53:40.710: D/AndroidRuntime(29458): Shutting down VM 08-30 19:53:40.710: W/dalvikvm(29458): threadid=1: thread exiting with uncaught exception (group=0x41a0c2a0) 
08-30 19:53:40.715: E/AndroidRuntime(29458): FATAL EXCEPTION: main 08-30 19:53:40.715: E/AndroidRuntime(29458): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.realscale.stefanrafa/com.realscale.stefanrafa.MainActivity}: java.lang.NullPointerException 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2110)        
08-30 19:53:40.715: E/AndroidRuntime(29458):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2135) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at android.app.ActivityThread.access$700(ActivityThread.java:140) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1237) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at android.os.Handler.dispatchMessage(Handler.java:99) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at android.os.Looper.loop(Looper.java:137) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at android.app.ActivityThread.main(ActivityThread.java:4921) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at java.lang.reflect.Method.invokeNative(Native Method) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at java.lang.reflect.Method.invoke(Method.java:511) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1038) 08-30 19:53:40.715: E/AndroidRuntime(29458):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:805) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at dalvik.system.NativeStart.main(Native Method) 

08-30 19:53:40.715: E/AndroidRuntime(29458): Caused by: java.lang.NullPointerException 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at com.realscale.stefanrafa.MainActivity.onCreate(MainActivity.java:19) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at android.app.Activity.performCreate(Activity.java:5188) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1094) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2074) 

08-30 19:53:40.715: E/AndroidRuntime(29458):    ... 11 more

Answer 1

Pressure is a float from 0 to 1. What it means is hardware-dependent. Even if there is a hardware pressure sensor, the map between the actual force and the return value of getPressure() still depends on the device. Also, some devices without a hardware pressure sensor simulate pressure from the size of the touch point, that is, they are assuming that more pressure means your finger flattens out. Other devices just return a constant value.

Also, force or weight (measured in Newtons) is not the same as mass (measured in grams). See http://en.wikipedia.org/wiki/Mass_versus_weight.

If you want help with your exception then you should post the source and the line where it is thrown. adb log is your friend.

EDIT:

You use pressure uninitialized. Its just declared but not set to anything, so it is null. Calling one of its methods is a NullPointerException. Only Views (widgets) can receive MotionEvents. You need to do something with your activity (e.g. inflate a layout) and then set a touch listener to one of the views of the layout:

someView.setOnTouchListener(new View.OnTouchListener() {
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        float pressure = event.getPressure();
    }
});