Return to Answer

• Suggest not storing the rows/columns sizes separately. SSOT / Single Source of Truth or DRY, Java+DRY. Just use the .length, and provide accessor methods if need be.

• Use final in method args, it will eliminate bugs like you have above, aliasing numbers incorrectly int the constructor

• Use an instance, not static

• Paranoia is the programmer's lifestyle: I also modified my code to do a deepCopy of the provided int[][] array, otherwise there is reference leakage, and the Matrix class would be unable to enforce its own invariants if caller code later modified the int[][] they passed in.

• I made my Matrix immutable (see final private numbers[][]) out of habit. This is a good practice, unless you come up with a good reason for a mutable implementation (wouldn't be surprising for performance reasons in matrices).

• Suggest not storing the rows/columns sizes separately. SSOT / Single Source of Truth or DRY, Java+DRY. Just use the .length, and provide accessor methods if need be.

• Use final in method args, it will eliminate bugs like you have above, aliasing numbers incorrectly int the constructor

• Use an instance, not static

• Paranoia is the programmer's lifestyle: I also modified my code to do a deepCopy of the provided int[][] array, otherwise there is reference leakage, and the Matrix class would be unable to enforce its own invariants if caller code later modified the int[][] they passed in.

• I made my Matrix immutable (see final private numbers[][]) out of habit. This is a good practice, unless you come up with a good reason for a mutable implementation (wouldn't be surprising for performance reasons in matrices).

• Suggest not storing the rows/columns sizes separately. SSOT / Single Source of Truth or DRY, Java+DRY. Just use the .length, and provide accessor methods if need be.
• Use final in method args, it will eliminate bugs like you have above, aliasing numbers incorrectly int the constructor
• Use an instance, not static
• Paranoia is the programmer's lifestyle: I also modified my code to do a deepCopy of the provided int[][] array, otherwise there is reference leakage, and the Matrix class would be unable to enforce its own invariants if caller code later modified the int[][] they passed in.

public final class Matrix { private int[][] numbers; // note the final, which would find a bug in your cited code above... public Matrix(final int[][] numbers) { // by enforcing these assumptions / invariants here, you don't need to deal // with checking them in other parts of the code. This is long enough that you might // factor it out into a private void sanityCheck() method, which could be // applied elsewhere when there are non-trivial mutations of the internal state if (numbers == null || numbers.length == 0) throw new NullPointerException("Matrix can't have null contents or zero rows"); final int columns = numbers[0].length; if (columns == 0) throw new IllegalArgumentException("Matrix can't have zero columns"); for (int i =1; i < numbers.length; i++) { if (numbers[i] == null) throw new NullPointerException("Matrix can't have null row "+i); if (numbers[i].length != columns) throw new IllegalArgumentException("Matrix can't have differing row lengths!"); } this.numbers = deepCopy(numbers); } public boolean isSquareMatrix() { return rowCount() == columnCount(); } public int rowCount() { return numbers.length; } public int columnCount() {return numbers[0].length; } private static int[][] deepCopy(final int[][] source) { // note we ignore error cases that don't apply because of // invariants in the constructor: assert(source != null); assert(source.length != 0); assert(source[0] != null); assert(source[0].length != 0); int[][] target = new int[source.length][source[0].length]; for (int i = 0; i < source.length; i++) target[i] = Arrays.copyOf(source[i],source[i].length); return target; } public Matrix getTranspose() { int[][] trans = new int[columnCount()][rowCount()]; for (int i = 0; i < rowCount(); i++) for (int j = 0; j < columnCount(); j++) trans[i][j] = getValue(j, i); return new Matrix(trans); } @Override public String toString() { StringBuilder sb = new StringBuilder(); for (int i = 0; i < numbers.length; i++) { for (int j = 0; j < numbers[i].length; j++) sb.append(' ').append(numbers[i][j]); sb.append('\n'); } return sb.toString(); } public static void main(String[] args) { final int[][] m1 = new int[][] { { 1, 4 }, { 5, 3 } }; Matrix mat = new Matrix(m1); System.out.print(mat); System.out.print(mat.getTranspose()); } } 

• Suggest not storing the rows/columns sizes separately. SSOT / Single Source of Truth or DRY, Java+DRY. Just use the .length, and provide accessor methods if need be.

• Use final in method args, it will eliminate bugs like you have above, aliasing numbers incorrectly int the constructor

• Use an instance, not static

• Paranoia is the programmer's lifestyle: I also modified my code to do a deepCopy of the provided int[][] array, otherwise there is reference leakage, and the Matrix class would be unable to enforce its own invariants if caller code later modified the int[][] they passed in.

• I made my Matrix immutable (see final private numbers[][]) out of habit. This is a good practice, unless you come up with a good reason for a mutable implementation (wouldn't be surprising for performance reasons in matrices).

public final class Matrix {   final private int[][] numbers; // note the final, which would find a bug in your cited code above... public Matrix(final int[][] numbers) { // by enforcing these assumptions / invariants here, you don't need to deal // with checking them in other parts of the code. This is long enough that you might // factor it out into a private void sanityCheck() method, which could be // applied elsewhere when there are non-trivial mutations of the internal state if (numbers == null || numbers.length == 0) throw new NullPointerException("Matrix can't have null contents or zero rows"); final int columns = numbers[0].length; if (columns == 0) throw new IllegalArgumentException("Matrix can't have zero columns"); for (int i =1; i < numbers.length; i++) { if (numbers[i] == null) throw new NullPointerException("Matrix can't have null row "+i); if (numbers[i].length != columns) throw new IllegalArgumentException("Matrix can't have differing row lengths!"); } this.numbers = deepCopy(numbers); } public boolean isSquareMatrix() { return rowCount() == columnCount(); } public int rowCount() { return numbers.length; } public int columnCount() {return numbers[0].length; } private static int[][] deepCopy(final int[][] source) { // note we ignore error cases that don't apply because of // invariants in the constructor: assert(source != null); assert(source.length != 0); assert(source[0] != null); assert(source[0].length != 0); int[][] target = new int[source.length][source[0].length]; for (int i = 0; i < source.length; i++) target[i] = Arrays.copyOf(source[i],source[i].length); return target; } public Matrix getTranspose() { int[][] trans = new int[columnCount()][rowCount()]; for (int i = 0; i < rowCount(); i++) for (int j = 0; j < columnCount(); j++) trans[i][j] = getValue(j, i); return new Matrix(trans); } @Override public String toString() { StringBuilder sb = new StringBuilder(); for (int i = 0; i < numbers.length; i++) { for (int j = 0; j < numbers[i].length; j++) sb.append(' ').append(numbers[i][j]); sb.append('\n'); } return sb.toString(); } public static void main(String[] args) { final int[][] m1 = new int[][] { { 1, 4 }, { 5, 3 } }; Matrix mat = new Matrix(m1); System.out.print(mat); System.out.print(mat.getTranspose()); } } 

You need to implement toString() in a meaningful way. For (untested) example:

@Override public String toString() { StringBuilder sb = new StringBuilder(); for (int i = 0; i < numbers.length; i++) { for (int j = 0; j < numbers[i].length; j++) sb.append(' ').append(numbers[i][j]); sb.append('\n'); } return sb.toString(); } 

This toString() is perhaps suitable for debugging, but will be ugly and confusing if you use it for real user output. An actual solution would probably use a Formatter in some complicated way to produce neatly tabular rows and columns.

public final class Matrix { private int[][] numbers; // note the final, which would find a bug in your cited code above... public Matrix(final int[][] numbers) { // by enforcing these assumptions / invariants here, you don't need to deal // with checking them in other parts of the code. This is long enough that you might // factor it out into a private void sanityCheck() method, which could be // applied elsewhere when there are non-trivial mutations of the internal state if (numbers == null || numbers.length == 0) throw new NullPointerException("Matrix can't have null contents or zero rows"); final int columns = numbers[0].length; if (columns == 0) throw new IllegalArgumentException("Matrix can't have zero columns"); for (int i =1; i < numbers.length; i++) { if (numbers[i] == null) throw new NullPointerException("Matrix can't have null row "+i); if (numbers[i].length != columns) throw new IllegalArgumentException("Matrix can't have differing row lengths!"); } this.numbers = deepCopy(numbers); } public boolean isSquareMatrix() { return rowCount() == columnCount(); } public int rowCount() { return numbers.length; } public int columnCount() {return numbers[0].length; } private static int[][] deepCopy(final int[][] source) { // note we ignore error cases that don't apply because of // invariants in the constructor: assert(source != null); assert(source.length != 0); assert(source[0] != null); assert(source[0].length != 0); int[][] target = new int[source.length][source[0].length]; for (int i = 0; i < source.length; i++) target[i] = Arrays.copyOf(source[i],source[i].length); return target; } public Matrix getTranspose() { int[][] trans = new int[columnCount()][rowCount()]; for (int i = 0; i < rowCount(); i++) for (int j = 0; j < columnCount(); j++) trans[i][j] = getValue(j, i); return new Matrix(trans); } public static void main(String[] args) { final int[][] m1 = new int[][] { { 1, 4 }, { 5, 3 } }; Matrix mat = new Matrix(m1); System.out.print(mat); System.out.print(mat.getTranspose()); } } 

You need to implement toString() in a meaningful way.

This toString() (below) is perhaps suitable for debugging, but will be ugly and confusing if you use it for real user output. An actual solution would probably use a Formatter in some complicated way to produce neatly tabular rows and columns.

public final class Matrix { private int[][] numbers; // note the final, which would find a bug in your cited code above... public Matrix(final int[][] numbers) { // by enforcing these assumptions / invariants here, you don't need to deal // with checking them in other parts of the code. This is long enough that you might // factor it out into a private void sanityCheck() method, which could be // applied elsewhere when there are non-trivial mutations of the internal state if (numbers == null || numbers.length == 0) throw new NullPointerException("Matrix can't have null contents or zero rows"); final int columns = numbers[0].length; if (columns == 0) throw new IllegalArgumentException("Matrix can't have zero columns"); for (int i =1; i < numbers.length; i++) { if (numbers[i] == null) throw new NullPointerException("Matrix can't have null row "+i); if (numbers[i].length != columns) throw new IllegalArgumentException("Matrix can't have differing row lengths!"); } this.numbers = deepCopy(numbers); } public boolean isSquareMatrix() { return rowCount() == columnCount(); } public int rowCount() { return numbers.length; } public int columnCount() {return numbers[0].length; } private static int[][] deepCopy(final int[][] source) { // note we ignore error cases that don't apply because of // invariants in the constructor: assert(source != null); assert(source.length != 0); assert(source[0] != null); assert(source[0].length != 0); int[][] target = new int[source.length][source[0].length]; for (int i = 0; i < source.length; i++) target[i] = Arrays.copyOf(source[i],source[i].length); return target; } public Matrix getTranspose() { int[][] trans = new int[columnCount()][rowCount()]; for (int i = 0; i < rowCount(); i++) for (int j = 0; j < columnCount(); j++) trans[i][j] = getValue(j, i); return new Matrix(trans); } @Override public String toString() { StringBuilder sb = new StringBuilder(); for (int i = 0; i < numbers.length; i++) { for (int j = 0; j < numbers[i].length; j++) sb.append(' ').append(numbers[i][j]); sb.append('\n'); } return sb.toString(); } public static void main(String[] args) { final int[][] m1 = new int[][] { { 1, 4 }, { 5, 3 } }; Matrix mat = new Matrix(m1); System.out.print(mat); System.out.print(mat.getTranspose()); } }