package amplifiersapp;

import java.util.ArrayList;

public class AmplifiersApp $\{$

$/**$

$*$ @param args

$*/$

public static void main$($String$[]$ args$)\{$

double r$1=4$;

double r$2=20$;

ArrayList amplifiers $=$ new ArrayList$()$;

amplifiers.add$($new InvertingAmplifier$($r$1,$ r$2))$;

amplifiers.add$($new NoninvertingAmplifier$($r$1,$ r$2))$;

amplifiers.add$($new VdivAmplifier$($r$1,$ r$2))$;

for $($int i $=0$; i $<$ amplifiers.size$()$; i$++)\{$

System.out.println$($amplifiers$.$get$($i$).$getDescription$()+"$

$"$

$+$ "Gain: $"+$ amplifiers.get$($i$).$getGain$())$;

$\}$

$\}$

public abstract class Amplifier $\{$

double R$1,$ R$2$;

public Amplifier$($double R$1,$ double R$2)\{$

this.R$1=$ R$1$;

this.R$2=$ R$2$;

$\}$

public abstract String getDescription$()$;

public abstract double getGain$()$;

$\}$

public class VdivAmplifier extends Amplifier $\{$

public VdivAmplifier$($double R$1,$ double R$2)\{$

super$($R$1,$ R$2)$;

$\}$

@Override

public String getDescription$()\{$

return "Vdiv Aplifiers: R$1="+$ R$1+"$ R$2="+$ R$2$;

$\}$

@Override

public double getGain$()\{$

return $(($R$2)/($R$1+$R$2))$;

$\}$

$\}$

public class NoninvertingAmplifier extends Amplifier $\{$

public NoninvertingAmplifier$($double R$1,$ double R$2)\{$

super$($R$1,$ R$2)$;

$\}$

@Override

public String getDescription$()\{$

return "Noninverting Aplifiers: R$1="+$ R$1+"$ R$2="+$ R$2$;

$\}$

@Override

public double getGain$()\{$

return $1+($R$2/$R$1)$;

$\}$

$\}$

public class InvertingAmplifier extends Amplifier $\{$

public InvertingAmplifier$($double R$1,$ double R$2)\{$

super$($R$1,$ R$2)$;

$\}$

@Override

public String getDescription$()\{$

return "Inverting Aplifiers: R$1="+$ R$1+"$ R$2="+$ R$2$;

$\}$

@Override

public double getGain$()\{$

return $(-($R$2/$R$1))$;

$\}$

$\}$

$\}$

I am getting this error error: non$-$static variable this cannot be referenced from a static context

amplifiers.add$($new InvertingAmplifier$($r$1,$ r$2))$;

Also public static void main$($String$[]$ args$)\{$

double r$1=4$;

double r$2=20$;

ArrayList amplifiers $=$ new ArrayList$()$;

amplifiers.add$($new InvertingAmplifier$($r$1,$ r$2))$;

amplifiers.add$($new NoninvertingAmplifier$($r$1,$ r$2))$;

amplifiers.add$($new VdivAmplifier$($r$1,$ r$2))$;

for $($int i $=0$; i $<$ amplifiers.size$()$; i$++)\{$

System.out.println$($amplifiers$.$get$($i$).$getDescription$()+"$

$"$

$+$ "Gain: $"+$ amplifiers.get$($i$).$getGain$())$;

$\}$

$\}$

this needs to be unchanged