为什么打印3 ln 8 4？

Question

boolean[][] values = new boolean[3][4];
System.out.println(values.length);
values[2] = new boolean[8];
System.out.println(values[2].length + " " + values[0].length);

This is a multiple choice question that I'm having trouble with. The answer is supposed to be:

3
8 4

but I thought that an array's size cannot be changed once it is created. Any explanations would be much appreciated.

Answer 1

You are not changing the length of an array, you are creating a new one of a different length.

values[2] = new boolean[4]; 
assert values[2].length == 4;

values[2] = new boolean[8]; 
assert values[2].length == 8;

is just like writing

boolean[] values2 = new boolean[4]; 
assert values2.length == 4;

values2 = new boolean[8]; 
assert values2.length == 8;

Note: a boolean[] variable is a reference to an array. It is not the array object so when you change this reference you point to a different object.

Answer 2

values = new boolean[3][4]

... creates an array of length 3, pointed to by a variable called values . Each of the three elements in values points to an array of length 4.

So:

System.out.println(values.length);

... prints 3.

values[2] = new boolean[8];

... creates a new array of length 8, and makes element 2 of values point to it.

The array that used to be element 2 of values no longer has a reference -- it's lost (if the JVM stays around long enough, it will be cleared away by garbage collection).

values[0] is still the 4 element array created at the start. values[2] is the newly created array of length 8.

Answer 3

In Java, two-dimensional arrays are actually arrays of arrays, not a block of bytes that's divided into rows, as it is in some other languages.

When you declare an array as

boolean[][] values = new boolean[3][4];

It's basically just a shorthand to writing:

boolean[][] values = {
     new boolean[4],
     new boolean[4],
     new boolean[4]
};

So you have an array whose elements are arrays of booleans.

There is nothing stopping you from changing one of the entries in this array. You have a first array, a second array, and a third array, and you are just replacing the third.

Thus, it is not changing the size of the array, because the declaration new boolean[3][4] only sets a fixed size for the first dimension, and for the initial values (arrays) in it. But you can replace those initial values with new values which have a different size if you wish - as long as you don't try to change the size of the main array.