In Android, how to make Login button disable with respect to EditText?

You need to implement TextWatcher on EditText to achieve the result.

EditText et1, et2;
Button b;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    et1 = (EditText) findViewById(R.id.editText1);
    et2 = (EditText) findViewById(R.id.editText2);
    b = (Button) findViewById(R.id.button1);

    checkValidation();

    et1.addTextChangedListener(mWatcher);
    et2.addTextChangedListener(mWatcher);
}

private void checkValidation() {
    // TODO Auto-generated method stub

    if ((TextUtils.isEmpty(et1.getText()))
            || (TextUtils.isEmpty(et2.getText())))
        b.setEnabled(false);
    else
        b.setEnabled(true);

}

TextWatcher mWatcher = new TextWatcher() {

    @Override
    public void onTextChanged(CharSequence s, int start, int before,
            int count) {
        // TODO Auto-generated method stub
        checkValidation();
    }

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count,
            int after) {
        // TODO Auto-generated method stub

    }

    @Override
    public void afterTextChanged(Editable s) {
        // TODO Auto-generated method stub

    }
};

try this:

EditText et1,et2;
Button b;

@Override
protected void onCreate(Bundle savedInstanceState) {

  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_login_check);
  et1 = (EditText) findViewById(R.id.editText1);
  et2 = (EditText) findViewById(R.id.editText2);
  b = (Button) findViewById(R.id.button1);


  et1.addTextChangedListener(new TextWatcher() {

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {

                            String s1 = et1.getText().toString();
                            String s2 = et2.getText().toString();

                             if(s1.equals("") && s2.equals("")){
                                   b.setEnabled(false);
                            } 
                            else if(!s1.equals("")&&s2.equals("")){
                                   b.setEnabled(false);
                            }
                            else if(!s2.equals("")&&s1.equals(""){
                                   b.setEnabled(false);
                            }
                            else {
                            b.setEnabled(true);
                          }

        }

        @Override
        public void beforeTextChanged(CharSequence s, int start, int count,
                int after) {
            // TODO Auto-generated method stub

        }

        @Override
        public void afterTextChanged(Editable s) {
            // TODO Auto-generated method stub

        }
    });
  et2.addTextChangedListener(new TextWatcher() {

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {

                            String s1 = et1.getText().toString();
                            String s2 = et2.getText().toString();

                            if(s1.equals("") && s2.equals("")){
                                   b.setEnabled(false);
                            } 
                            else if(!s1.equals("")&&s2.equals("")){
                                   b.setEnabled(false);
                            }
                            else if(!s2.equals("")&&s1.equals(""){
                                   b.setEnabled(false);
                            }
                            else {
                            b.setEnabled(true);
                          }

        }

        @Override
        public void beforeTextChanged(CharSequence s, int start, int count,
                int after) {
            // TODO Auto-generated method stub

        }

        @Override
        public void afterTextChanged(Editable s) {
            // TODO Auto-generated method stub

        }
    });

}

You need to track user's action inside EditText using TextWatcher object:

myEditText.addTextChangedListener(new TextWatcher()
        {

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count)
            {

            }

            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after)
            {

            }

            @Override
            public void afterTextChanged(Editable s)
            {
                if (s.length() > 1)
                {
                    //enable button
                } else
                    //disable
            }
        });

heres what you are looking for :

private EditText et1,et2;
//  create a textWatcher member
private TextWatcher mTextWatcher = 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) {
    }

    @Override
    public void afterTextChanged(Editable editable) {
        // check Fields For Empty Values
        checkFieldsForEmptyValues();
    }
};

void checkFieldsForEmptyValues(){
    Button b = (Button) findViewById(R.id.button1);

    String s1 = et1.getText().toString();
    String s2 = et2.getText().toString();

    if(s1.equals("")|| s2.equals("")){
        b.setEnabled(false);
    } else {
        b.setEnabled(true);
    }
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login_check);
    et1 = (EditText) findViewById(R.id.editText1);
    et2 = (EditText) findViewById(R.id.editText2);


    // set listeners
    et1.addTextChangedListener(mTextWatcher);
    et2.addTextChangedListener(mTextWatcher);

    // run once to disable if empty
    checkFieldsForEmptyValues(); 
}