import android.os.Bundle;
    import android.support.v7.app.AppCompatActivity;
    import android.view.View;
    import android.widget.Button;
    import android.widget.CheckBox;
    import android.widget.RadioButton;
    import android.widget.TextView;

    import java.util.Random;


    public class MainActivity extends AppCompatActivity {

    Button btnGenPassword;
    Button btnClear;
    TextView lblResult;
    RadioButton radioCurrent;
    CheckBox checkUpper;
    CheckBox checkLower;
    CheckBox checkSpecial;
    CheckBox checkNumber;
    int pwLength;
    String validChars = "";

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

        btnGenPassword = (Button) findViewById(R.id.btnGeneratePassword);
        btnClear = (Button) findViewById(R.id.btnClear);
        lblResult = (TextView) findViewById(R.id.lblResult);

        checkLower = (CheckBox) findViewById(R.id.checkLower);
        checkUpper = (CheckBox) findViewById(R.id.checkUpper);
        checkSpecial = (CheckBox) findViewById(R.id.checkSpecial);
        checkNumber = (CheckBox) findViewById(R.id.checkNumber);

        //Start the radio
        radioCurrent = (RadioButton) findViewById(R.id.radio_8);
        radioCurrent.isChecked();
        pwLength = 8;

        //set up onClick sending result to textfield
        btnGenPassword.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String result = passwordGen();
                lblResult.setText(result);
                CheckBox();

            }
        });

        //Clear result
        btnClear.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                lblResult.setText("");
            }
        });
    }

    public String passwordGen() {
        char[] usableChars = validChars.toCharArray();
        String password = new String();
        Random generator = new Random();

        for (Integer i = 0; i < pwLength; i++) {
            password = password + usableChars[generator.nextInt(usableChars.length)];
        }

        return password;

    }
    //Radio Button
    public void onRadioButtonClicked(View view) {
        //Is the button now checked
        boolean checked = ((RadioButton) view).isChecked();
        //check which radio button was clicked
        switch (view.getId()) {
            case R.id.radio_8:
                if (checked) {
                    pwLength = 8;
                }
                break;
            case R.id.radio_10:
                if (checked) {
                    pwLength = 10;
                }
                break;
            case R.id.radio_12:
                if (checked) {
                    pwLength = 12;
                }
                break;
        }
    }

    public void CheckBox() {

        validChars = "";
        // checkbox checked and and listing valid characters
        if (checkNumber.isChecked()) {
            validChars = "123456789";
        }
        if (checkSpecial.isChecked()) {
            validChars = "!@#$%^&*()";
        }
        if (checkLower.isChecked()) {
            validChars = "abcdefghijklmnopqrstuvwxyz";
        }
        if (checkUpper.isChecked()) {
            validChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        }
    }

}

Here is my xml file

 <EditText
        android:id="@+id/lblResult"
        android:layout_width="368dp"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="text"
        tools:layout_editor_absoluteX="8dp"
        tools:layout_editor_absoluteY="16dp" />


    <RadioGroup
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <RadioButton
            android:id="@+id/radio_8"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="20sp"
            android:layout_weight="1"
            android:onClick="onRadioButtonClicked"
            android:text="8 Char"
            tools:layout_editor_absoluteX="8dp"
            tools:layout_editor_absoluteY="77dp" />

        <RadioButton
            android:id="@+id/radio_10"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:onClick="onRadioButtonClicked"
            android:text="10 Char"
            tools:layout_editor_absoluteX="139dp"
            tools:layout_editor_absoluteY="77dp" />

        <RadioButton
            android:id="@+id/radio_12"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:onClick="onRadioButtonClicked"
            android:text="12 Char"
            tools:layout_editor_absoluteX="287dp"
            tools:layout_editor_absoluteY="77dp" />

    </RadioGroup>

    <TableRow
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="horizontal"
        tools:layout_editor_absoluteX="173dp"
        tools:layout_editor_absoluteY="192dp">

        <CheckBox
            android:id="@+id/checkNumber"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="60sp"
            android:text="Numbers"
            tools:layout_editor_absoluteX="76dp"
            tools:layout_editor_absoluteY="118dp" />

        <EditText
            android:id="@+id/editText2"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:background="@null"
            android:selectAllOnFocus="false" />

        <CheckBox
            android:id="@+id/checkSpecial"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="35sp"
            android:text="Spec Char"
            tools:layout_editor_absoluteX="210dp"
            tools:layout_editor_absoluteY="119dp" />

        <EditText
            android:id="@+id/editText4"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:background="@null"
            android:selectAllOnFocus="false" />
    </TableRow>

    <TableRow
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:layout_editor_absoluteX="192dp"
        tools:layout_editor_absoluteY="255dp"
        android:orientation="horizontal">

        <CheckBox
            android:id="@+id/checkLower"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="60sp"
            android:text="Lower Case"
            tools:layout_editor_absoluteX="210dp"
            tools:layout_editor_absoluteY="160dp" />

        <EditText
            android:id="@+id/editText3"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:background="@null" />

        <CheckBox
            android:id="@+id/checkUpper"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="35sp"
            android:text="Upper Case"
            tools:layout_editor_absoluteX="76dp"
            tools:layout_editor_absoluteY="160dp" />
    </TableRow>

    <Button
        android:id="@+id/btnGeneratePassword"
        android:layout_marginTop="5dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="CheckBox"
        android:background="?android:attr/colorActivatedHighlight"
        android:text="Generate Password" />

    <Button
        android:id="@+id/btnClear"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="10sp"
        android:background="?android:attr/colorActivatedHighlight"
        android:onClick="onRadioButtonClicked"
        android:text="Clear" />
</TableLayout>