Custom Button Widget which applies a ripple onto buttons above API > 21 and normal state list selectors for API < 21. Also you can set the corner radius.

BaseButton.java

package com.mohitkanwal.framework.control.base;

import android.annotation.TargetApi;
import android.content.Context;
import android.content.res.ColorStateList;
import android.content.res.TypedArray;
import android.graphics.Color;
import android.graphics.drawable.RippleDrawable;
import android.graphics.drawable.ShapeDrawable;
import android.graphics.drawable.StateListDrawable;
import android.os.Build;
import android.util.AttributeSet;
import android.widget.TextView;

/**
 * User: msk
 * Date: 13/01/2015
 */
public class BaseButton extends TextView {
    private float cornerRadius = UIValueHelper.COMMON_VALUE.dp2;

    private int normalColor = Color.BLUE;

    private int disabledColor = Color.LTGRAY;

    public BaseButton(Context context) {
        super(context);
        init(context, null);
    }

    public BaseButton(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(context, attrs);
    }

    public BaseButton(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init(context, attrs);
    }

    private static ColorStateList getPressedColorSelector(int normalColor, int pressedColor, int disabledColor) {
        return new ColorStateList(
                new int[][]{
                        new int[]{android.R.attr.state_pressed},
                        new int[]{android.R.attr.state_focused},
                        new int[]{android.R.attr.state_activated},
                        new int[]{}
                },
                new int[]{
                        pressedColor,
                        pressedColor,
                        pressedColor,
                        pressedColor
                }
        );
    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    private void init(final Context context, final AttributeSet attrs) {
        if (attrs != null) {
            TypedArray typedArray = context.obtainStyledAttributes(attrs, R.styleable.BaseButton);
            normalColor = typedArray.getColor(R.styleable.BaseButton_normalStateColor, normalColor);
            disabledColor = typedArray.getColor(R.styleable.BaseButton_disabledStateColor, disabledColor);
            cornerRadius = typedArray.getDimension(R.styleable.BaseButton_cornerRadius, cornerRadius);
            typedArray.recycle();
        }

        final int pressedColor = UIUtils.darkenColor(normalColor, 0.8f);

        final ShapeDrawable normalDrawable = UIUtils.makeRectShapeWithColor(normalColor, cornerRadius);

        if (VersionUtils.isLollipopAndAbove()) {
            final RippleDrawable rippleDrawable = new RippleDrawable(getPressedColorSelector(normalColor, pressedColor, disabledColor), normalDrawable, null);
            setBackground(rippleDrawable);
        } else {
            final StateListDrawable states = new StateListDrawable();
            states.addState(new int[]{android.R.attr.state_pressed}, UIUtils.makeRectShapeWithColor(pressedColor, cornerRadius));
            states.addState(new int[]{-android.R.attr.state_enabled}, UIUtils.makeRectShapeWithColor(disabledColor, cornerRadius));
            states.addState(new int[]{}, normalDrawable);
            setBackground(states);
        }
    }
}

UIUtils.java

package com.mohitkanwal.framework.helper;

import android.annotation.SuppressLint;
import android.graphics.*;
import android.graphics.drawable.Drawable;
import android.graphics.drawable.ShapeDrawable;
import android.graphics.drawable.shapes.RectShape;
import android.support.annotation.ColorInt;
import android.view.View;
import android.view.Window;
import android.widget.AbsListView;
import android.widget.ImageView;

import java.util.Locale;

/**
 * A utility class for UI related helper functions.
 * User: mohit
 * Date: 18/12/14
 */
public final class UIUtils {

    private UIUtils() {
    }
    
    /**
     * Darkens or lightens a color by a given fraction.
     *
     * @param color    The color to be darkened or lightened.
     * @param fraction A value below 1.0 to darken, a value above 1.0 to lighten.
     * @return The darkened or lightened color.
     */
    public static int darkenColor(final int color, final float fraction) {

        final int red = Math.round(Math.min(Color.red(color) * fraction, 255));
        final int green = Math.round(Math.min(Color.green(color) * fraction, 255));
        final int blue = Math.round(Math.min(Color.blue(color) * fraction, 255));

        final int alpha = Color.alpha(color);

        return Color.argb(alpha, red, green, blue);
    }

    
    /**
     * Makes a rect shape with the given color and rounded corner radius.
     *
     * @param color  The color of the shape.
     * @param radius The rounded corner radius of the shape.
     * @return The shape.
     */
    public static ShapeDrawable makeRectShapeWithColor(final @ColorInt int color, final float radius) {
        final ShapeDrawable drawable = new ShapeDrawable(new RectShape());
        final Paint paint = drawable.getPaint();
        paint.setColor(color);
        paint.setStyle(Paint.Style.FILL_AND_STROKE);
        paint.setStrokeWidth(0);
        if (radius > 0f) {
            paint.setPathEffect(new CornerPathEffect(radius));
        }
        paint.setDither(true);
        paint.setAntiAlias(true);

        return drawable;
    }
}

usage.xml

<com.youapp.BaseButton
                android:id="@+id/btn"
                android:layout_width="match_parent"
                android:layout_height="48dp"
                android:layout_marginTop="24dp"
                app:cornerRadius="2dp"
                app:disabledStateColor="@color/lightGray"
                app:normalStateColor="@color/colorPrimary"/>