Tiny app to demonstrate one way of using FlexboxLayoutManager

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.RecyclerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/recycler"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

itemview.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="80dp"
    android:layout_height="48dp"
    android:layout_margin="4dp">

    <TextView
        android:id="@+id/text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:textSize="22sp"/>

</android.support.v7.widget.CardView>

MainActivity.java

package com.example.stackoverflow;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import com.google.android.flexbox.FlexDirection;
import com.google.android.flexbox.FlexboxLayoutManager;
import com.google.android.flexbox.JustifyContent;

public class MainActivity extends AppCompatActivity {

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

        FlexboxLayoutManager manager = new FlexboxLayoutManager(this, FlexDirection.ROW);
        manager.setJustifyContent(JustifyContent.CENTER);

        RecyclerView recycler = (RecyclerView) findViewById(R.id.recycler);
        recycler.setLayoutManager(manager);
        recycler.setAdapter(new MyAdapter());
    }

    private static class MyAdapter extends RecyclerView.Adapter<MyViewHolder> {

        @Override
        public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            LayoutInflater inflater = LayoutInflater.from(parent.getContext());
            View itemView = inflater.inflate(R.layout.itemview, parent, false);

            ViewGroup.MarginLayoutParams layoutParams = (ViewGroup.MarginLayoutParams) itemView.getLayoutParams();
            layoutParams.width = (parent.getWidth() / 4) - layoutParams.leftMargin - layoutParams.rightMargin;
            itemView.setLayoutParams(layoutParams);

            return new MyViewHolder(itemView);
        }

        @Override
        public void onBindViewHolder(MyViewHolder holder, int position) {
            holder.textView.setText("" + position);
        }

        @Override
        public int getItemCount() {
            return 6;
        }
    }

    private static class MyViewHolder extends RecyclerView.ViewHolder {

        private final TextView textView;

        public MyViewHolder(View itemView) {
            super(itemView);
            this.textView = (TextView) itemView.findViewById(R.id.text);
        }
    }
}

Note that in itemview.xml line 4, the value of android:layout_width is irrelevant, since it will be overwritten in Java code.