Java - Trying to put null values on array

Question

I'm trying to put null values on some values of this array:

public ElectricInstallation(){
    this.dim1 = (int) Math.random()*85 + 1;
    this.dim2 = (int) Math.random()*85 + 1;
    this.dim3 = (int) Math.random()*85 + 1;
    for(int i = 0; i < this.dim1; i++){
        for(int j = 0; j < this.dim2; j++){
            for(int k = 0; k < this.dim3; k++){
                this.machine[i][j][k] = null;
            }
        }
    }
}

Ergo, I'm trying to create empty values on this array to do this after that:

public void makeScenario(){
    for(int i = 0; i < this.dim1; i++){
        for(int j = 0; j < this.dim2; j++){
            for(int k = 0; k < this.dim3; k++){
                if(Math.random() < 0.7){
                    this.machines[i][j][k] = new ElectricMachine((int) Math.random()*15000 + 1);
                    if(Math.random() < 0.5){
                        this.machine[i][j][k].clic();
                    }
                }
            }
        }
    }
}

In other words, I want to make some values to be ElectricMachine and others to be null, but java throws me this: "java.lang.NullPointerException".

What can I do?

Pierre Rust · Accepted Answer · 2014-02-27 12:33:07Z

It seems you did non initialized your array, and only declared it.

You should probably initialize it in the constructor, has it seems you choosing it's size there :

public ElectricInstallation(){
    this.dim1 = (int) Math.random()*85 + 1;
    this.dim2 = (int) Math.random()*85 + 1;
    this.dim3 = (int) Math.random()*85 + 1;

    // I think this is the missing part : 
    this.machine = new ElectricMachine[dim1][dim2][dim3];

    for(int i = 0; i < this.dim1; i++){
        for(int j = 0; j < this.dim2; j++){
            for(int k = 0; k < this.dim3; k++){
                this.machine[i][j][k] = null;
            }
        }
    }
}

You also forget to post the stacktrace so we can see where the NullPointerException really happens. If you have initialized your array in some other code you have not posted, the NPE could well be inside the click() call.

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