HitungPersegi

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/main_acticity"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_margin="@dimen/medium_dimen"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/tv_hitung_persegi"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/hitung_persegi"
        android:textAlignment="center"
        android:textSize="30sp"
        android:textStyle="bold" />

    <EditText
        android:id="@+id/edt_sisi"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="@dimen/medium_dimen"
        android:layout_marginBottom="@dimen/small_dimen"
        android:hint="@string/masukkan_sisi_persegi"
        android:textSize="20sp" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/btn_hitung"
        android:text="@string/hitung"
        android:textAlignment="center"
        android:textStyle="bold"
        android:drawableLeft="@drawable/ic_flash_on_black_24dp"
        android:layout_gravity="center"
        android:layout_marginTop="@dimen/small_dimen"
        android:layout_marginBottom="@dimen/medium_dimen"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Keliling: 0"
        android:textSize="30sp"
        android:layout_margin="@dimen/medium_dimen"
        android:textStyle="bold"
        android:layout_gravity="center"
        android:id="@+id/tv_keliling"
        android:textColor="@color/colorPrimary"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Luas: 0"
        android:textSize="30sp"
        android:textStyle="bold"
        android:layout_gravity="center"
        android:id="@+id/tv_luas"
        android:textColor="@color/colorPrimary"/>
    
</LinearLayout>

MainActivity.java

package com.example.hitungpersegi;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    EditText edtSisi;
    Button btnHitung;
    TextView tvKeliling, tvLuas;

    double sisi, keliling, luas;


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

        edtSisi = findViewById(R.id.edt_sisi);
        btnHitung = findViewById(R.id.btn_hitung);
        tvKeliling = findViewById(R.id.tv_keliling);
        tvLuas = findViewById(R.id.tv_luas);

        btnHitung.setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        if (view.getId() == R.id.btn_hitung) {
            if (edtSisi.getText().toString() == null || edtSisi.getText().toString().isEmpty()) {
                sisi = 0.0;
            } else {
                sisi = Double.parseDouble(edtSisi.getText().toString());
            }

            keliling = 4 * sisi;
            luas = sisi * sisi;

            tvKeliling.setText("Keliling: " + keliling);
            tvLuas.setText("Luas: " + luas);
        }
    }

    @Override
    protected void onStart() {
        super.onStart();

        luas = 0.0;
        keliling = 0.0;

        Toast.makeText(this, "Activity dalam kondisi onStart()", Toast.LENGTH_LONG).show();
        Log.d("onStart: inisialisasi nilai luas dan keliling serta memunculkan Toast", String.valueOf(luas));
    }

    @Override
    protected void onStop() {
        super.onStop();

        Toast.makeText(this, "Activity ditutup.", Toast.LENGTH_LONG).show();
    }
}