Java Nullpointexcepeption

Question

I'm getting a null point error, but I don't know why. Below are the classes which I use.

Kofferslot.java

package com.tvh;

/**
 * @author 
 */
public class Kofferslot {
    private static Letter[] letterlijst;
    private static Cijfer cijfer = new Cijfer();

    Kofferslot(){
        letterlijst[0] = new Letter();
        letterlijst[1] = new Letter();

        Cijfer cijfer = new Cijfer();

    }

    Kofferslot(char firstLetter, char secondLetter, int cijfer){
        letterlijst[0].setLetter(firstLetter);
        letterlijst[1].setLetter(secondLetter);

        this.cijfer.setCijfer(cijfer);
    }

    public static String volgende(){

        letterlijst[0].volgende();
        char res1 = letterlijst[0].getLetter();

        letterlijst[1].volgende();
        char res2 = letterlijst[1].getLetter();

        cijfer.volgende();
        int rescijfer = cijfer.getCijfer();

        String result = String.valueOf(res1 + res2 + rescijfer);

        return result;

    }

    public static void main(String args[]){
        Kofferslot.volgende();
    }

}

Letter.java

package com.tvh;

/**
 * @author 
 */
public class Letter {
    private char letter;

    Letter() {
        char letter = 'A';
    }

    char getLetter() {
        return letter;
    }

    void setLetter(char letter) {
        this.letter = letter;

    }

    void volgende() {
        if (this.letter > 'Z') {
            letter = 'A';
        } else {
            letter++;
        }
    }

    public static void main(String[] args) {
        Letter letter = new Letter();
        letter.setLetter('A');
        letter.volgende();

        char letter1 = letter.getLetter();

        letter.setLetter('B');
        letter.volgende();

        char letter2 = letter.getLetter();

        System.out.print(letter1);
        System.out.print(letter2);
    }
}


package com.tvh;

/**
 * @author 
 */
public class Cijfer {
    private  static int cijfer;

    public Cijfer(){
        cijfer = 0;
    }

    public static int getCijfer(){
        return cijfer;

    }

    public void setCijfer(int cijfer){
        this.cijfer = cijfer;

    }

    public void volgende(){
        cijfer++;
    }

    public static void main(String[] args){
        System.out.println("Get cijfer object: " + Cijfer.getCijfer());

        Cijfer test = new Cijfer();
        System.out.println("New object: " + Cijfer.getCijfer());

    }


}

Stacktrace:

Exception in thread "main" java.lang.NullPointerException
    at com.tvh.Kofferslot.volgende(Kofferslot.java:26)
    at com.tvh.Main.main(Main.java:6)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(De`enter code here`legatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

I hope that you guys can help me finding the nullpointexception!

Thanks in advance! Because for now I'm stuck..