Arrays of objects exercise 1 solution

Complete the Arrays of objects exercises before reviewing the solutions.

Review the Arrays of objects exercise 1 solution with AP CS Tutor Brandon Horn.

Original code

Coordinate2D[] coors = new Coordinate2D[3];
coors[0] = new Coordinate2D(1, 1);
coors[1] = new Coordinate2D(2, 2);
coors[2] = new Coordinate2D(3, 3);

System.out.println(Arrays.toString(coors));
// prints: [(1, 1), (2, 2), (3, 3)]

Coordinate2D[] coors2 = new Coordinate2D[3];

for(int i = 0; i < coors2.length; i++)
    coors2[i] = coors[i];

coors2[0] = new Coordinate2D(4, 4);

coors2[1].setX(5);
coors2[1].setY(5);

System.out.println(Arrays.toString(coors));

System.out.println(Arrays.toString(coors2));

Output

[(1, 1), (2, 2), (3, 3)]
[(1, 1), (5, 5), (3, 3)]
[(4, 4), (5, 5), (3, 3)]

Explanation

An array of objects is really an array of references (memory addresses). (More generally, an array is a bunch of values of the same type stored in sequential memory locations.)

The statement coors2[i] = coors[i]; copies the memory address stored at coors[i] into coors2[i]. It does not copy the object to which coors[i] refers.

At the end of the for loop, both coors and coors2 refer to the same 3 objects. The Coordinate2D objects themselves are neither copied nor changed. This is known as creating a shallow copy of the array.

The statement coors2[0] = new Coordinate2D(4, 4); creates a new Coordinate2D object representing (4, 4) and stores its memory address in coors2[0]. This has no effect on coors, coors[0], or the object to which coors[0] refers.

The statements coors2[1].setX(5); and coors2[1].setY(5); run the mutator methods setX and setY on the object to which coors2[1] refers. coors[i] refers to the same object. The values being changed are those of the instance variables x and y inside the object. The values of coors2[1] and coors[1] (the memory address of the object) remain unchanged.

Step by step memory diagram

Step 1

Coordinate2D[] coors = new Coordinate2D[3];
coors[0] = new Coordinate2D(1, 1);
coors[1] = new Coordinate2D(2, 2);
coors[2] = new Coordinate2D(3, 3);

System.out.println(Arrays.toString(coors));
// prints: [(1, 1), (2, 2), (3, 3)]

Coordinate2D[] coors2 = new Coordinate2D[3];

// additional code not yet run

Memory diagram after Step 1

coors points to a box representing an array. The array points to 3 boxes representing Coordinate2D objects. The objects contain x: 1, y: 1, x: 2, y: 2, and x:3, y: 3. coors2 points to a different box representing a different array. The array contains 3 null values.

The default value for elements of an array of objects (really an array of memory addresses) is null.

Output after Step 1

[(1, 1), (2, 2), (3, 3)]

Step 2

Coordinate2D[] coors = new Coordinate2D[3];
coors[0] = new Coordinate2D(1, 1);
coors[1] = new Coordinate2D(2, 2);
coors[2] = new Coordinate2D(3, 3);

System.out.println(Arrays.toString(coors));
// prints: [(1, 1), (2, 2), (3, 3)]

Coordinate2D[] coors2 = new Coordinate2D[3];

for(int i = 0; i < coors2.length; i++)
    coors2[i] = coors[i];

// additional code not yet run

Memory diagram after Step 2

coors points to a box representing an array. coors2 points to a different box representing a different array. Box arrays points to the same 3 boxes, each representing a Coordinate2D object. The values inside the objects are x: 1, y: 1, x: 2, y: 2, and x:3, y: 3

The statement coors2[i] = coors[i]; copies the memory address stored at coors[i] into coors2[i]. It does not copy the object to which coors[i] refers.

At the end of the for loop, both coors and coors2 refer to the same 3 objects. The Coordinate2D objects themselves are neither copied nor changed. This is known as creating a shallow copy of the array.

Step 3

Coordinate2D[] coors = new Coordinate2D[3];
coors[0] = new Coordinate2D(1, 1);
coors[1] = new Coordinate2D(2, 2);
coors[2] = new Coordinate2D(3, 3);

System.out.println(Arrays.toString(coors));
// prints: [(1, 1), (2, 2), (3, 3)]

Coordinate2D[] coors2 = new Coordinate2D[3];

for(int i = 0; i < coors2.length; i++)
    coors2[i] = coors[i];

coors2[0] = new Coordinate2D(4, 4);

// additional code not yet run

Memory diagram after Step 3

coors points to a box representing an array. The array points to boxes representing 3 Coordinate2D objects. The objects contain the values x: 1, y: 1, x: 2, y: 2, and x: 3, y: 3. coors2 points to a different box representing a different array. The first value in coors2 points to a box representing a Coordinate2D object with values x: 4, y: 4. The other 2 values point to the same last 2 objects as coors.

The statement coors2[0] = new Coordinate2D(4, 4); creates a new Coordinate2D object representing (4, 4) and stores its memory address in coors2[0]. This has no effect on coors, coors[0], or the object to which coors[0] refers.

Step 4

Coordinate2D[] coors = new Coordinate2D[3];
coors[0] = new Coordinate2D(1, 1);
coors[1] = new Coordinate2D(2, 2);
coors[2] = new Coordinate2D(3, 3);

System.out.println(Arrays.toString(coors));
// prints: [(1, 1), (2, 2), (3, 3)]

Coordinate2D[] coors2 = new Coordinate2D[3];

for(int i = 0; i < coors2.length; i++)
    coors2[i] = coors[i];

coors2[0] = new Coordinate2D(4, 4);

coors2[1].setX(5);
coors2[1].setY(5);

System.out.println(Arrays.toString(coors));

System.out.println(Arrays.toString(coors2));

Memory diagram after Step 4

coors and coors2 point to different boxes representing different arrays. The first value in coors is a reference to an object containing x: 1, y: 1. The first value in coors2 is a reference to an object containing x: 4, y: 4. The 2nd and 3rd values in both coors and coors2 are references to objects containing x: 5, y: 5 and x: 3, y: 3, respectively

The statements coors2[1].setX(5); and coors2[1].setY(5); run the mutator methods setX and setY on the object to which coors2[1] refers. coors[i] refers to the same object. The values being changed are those of the instance variables x and y inside the object. The values of coors2[1] and coors[1] (the memory address of the object) remain unchanged.

Output after Step 4

[(1, 1), (2, 2), (3, 3)]
[(1, 1), (5, 5), (3, 3)]
[(4, 4), (5, 5), (3, 3)]

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