在 android 工作室中更改 imageview 的坐标时出错

Question

I am a beginner to android. I want to slide a slidebar in a horizontal linear layout but when I run the code, the application ends giving error FallingBall keeps stopping

My xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 
    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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


<LinearLayout
    android:id="@+id/linearLayout2"
    android:layout_width="0dp"
    android:layout_height="0dp"
    android:orientation="vertical"
    android:weightSum="100"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent">

    <LinearLayout
        android:id="@+id/linearLayout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="72"
        android:orientation="horizontal" >

        <TextView
            android:id="@+id/textView2"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:text="Hello" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_weight="8"
        android:orientation="horizontal">

        <ImageView
            android:id="@+id/slideBar"
            android:layout_width="100dp"
            android:layout_height="match_parent"
            app:srcCompat="@android:drawable/bottom_bar" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_weight="20"
        android:orientation="horizontal">

        <androidx.constraintlayout.widget.ConstraintLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent">

            <ImageButton
                android:id="@+id/leftBut"
                android:layout_width="wrap_content"
                android:layout_height="match_parent"
                android:background="@android:color/white"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toTopOf="parent"
                app:srcCompat="@drawable/left_but"
                android:onClick="hitLeft"/>

            <ImageButton
                android:id="@+id/rightBut"
                android:layout_width="wrap_content"
                android:layout_height="match_parent"
                android:background="@android:color/white"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintTop_toTopOf="parent"
                app:srcCompat="@drawable/right_but"
                android:onClick="hitRight"/>
        </androidx.constraintlayout.widget.ConstraintLayout>

    </LinearLayout>
</LinearLayout>

</androidx.constraintlayout.widget.ConstraintLayout>

My java file is like this:

    package com.example.fallingball;

import androidx.appcompat.app.AppCompatActivity;

import android.content.pm.ActivityInfo;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageButton;
import android.widget.ImageView;

public class MainActivity extends AppCompatActivity {

ImageView s = findViewById(R.id.slideBar);
float x = s.getX();
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);
}


void hitLeft(View view){
    x--;
    s.setX(30);
}

void hitRight(View view){
    x++;
    s.setX(40);
}
}

Note: leftBut and rightBut are two Image button and I have copied the images to the drawable folder so if you are trying this code on your system, make sure you adjust this else it will show you Image not found or so...

Answer 1

In MainActivity.java you should initialize your value into onCreate() method

ImageView s;
float x;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);
    s = findViewById(R.id.slideBar);
    x = s.getX();
}

Answer 2

Well, I found my own mistake. The thing what hossein answered earlier was right but the application was still crashing. Actually the problem was coming that I was triggering the methods hitLeft and hitRight which were not initialized as public . That was the mistake and now it is working alright...