ArrayStoreException while trying to copy array in java

Question

I have a problem with changing the length of the 2d array in java. After allocating space for the 2d array I can't copy old array's values to the new array. But I can do it on 1d arrays with similar code. Here is the working code:

public static Object[] changeLength1D(Object [] a, int n, int new_length){

    if(n > new_length){
        throw new IllegalArgumentException("n must be greater or equal to new_length");
    }

    // Allocate space for 1d array
    Object[] new_array = (Object[]) Array.newInstance(a.getClass().getComponentType(), new_length);
    // Efficient array copy from a[0:n-1] to new_array
    System.arraycopy(a, 0, new_array, 0, n);

    return new_array;
}

But same logic is not working here. When I use arraycopy , java throws this:

Exception in thread "main" java.lang.ArrayStoreException
at java.base/java.lang.System.arraycopy(Native Method)

Here is the code for 2d array:

public static Object[][] changeLength2D(Object [][] a, int dim1_limit, int dim2_limit,int dim1_newLength, int dim2_newLength){

    if(dim1_limit > dim1_newLength || dim2_limit > dim2_newLength){
        throw new IllegalArgumentException("Limits must be <= new lengths");
    }

    // Allocate space for 2d array
    Object[][] new_array = (Object[][]) Array.newInstance(a.getClass().getComponentType(),
            dim1_newLength,dim2_newLength);


    // Copy by rows
    for(int x = 0; x < dim1_limit; x++){
       System.arraycopy(a[x], 0, new_array[x], 0 ,dim2_limit); // EXCEPTION THROWS RIGHT THIS LINE
    }

    return new_array;
}

Actually i did before i asked the question but i already implement this solution and this is not working for my problem. When i assign the old array's values to new array, a type issue is occuring. — Mirac B.
– Mirac B., Commented Feb 14, 2018 at 7:17

xingbin · Accepted Answer · 2018-02-14 17:11:07Z

0

Reason

From the doc of Array.newInstance():

public static Object newInstance(Class<?> componentType,int... dimensions)
                      throws IllegalArgumentException,
                             NegativeArraySizeException

If componentType represents an array class, the number of dimensions of the new array is equal to the sum of dimensions.length and the number of dimensions of componentType

Since you create 2d array by:

Object[][] new_array = (Object[][]) Array.newInstance(a.getClass().getComponentType(),
        dim1_newLength, dim2_newLength);

Considering a is an array, new_array will have three dimensions, new_array[x] will have two dimensions, which will cause ArrayStoreException when running System.arrayCopy() because type mismatch.

Solution

create new 2d array with below and make sure a[0][0] not null

Object[][] new_array = (Object[][]) Array.newInstance(a[0][0].getClass(),
        dim1_newLength, dim2_newLength);

edited Feb 14, 2018 at 17:11

answered Feb 14, 2018 at 16:53

xingbin

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1 Comment

Mirac B. Over a year ago

Thanks a lot for explanation and reference link.

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