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## Concatenate Java Arrays

To concatenate arrays of Java® objects, use the MATLAB® `cat` function or the square bracket (`[]`) operators.

You can concatenate Java objects only along the first (vertical) or second (horizontal) axis. For more information, see How MATLAB Represents Java Arrays.

### Two-Dimensional Horizontal Concatenation

This example horizontally concatenates two Java arrays. Create 2-by-3 arrays `d1` and `d2`.

```d1 = javaArray('java.lang.Double',2,3); for m = 1:2 for n = 1:3 d1(m,n) = java.lang.Double(n*2 + m-1); end end d1```
```d1 = java.lang.Double[][]: [2] [4] [6] [3] [5] [7]```
```d2 = javaArray('java.lang.Double',2,2); for m = 1:2 for n = 1:3 d2(m,n) = java.lang.Double((n+3)*2 + m-1); end end d2```
```d2 = java.lang.Double[][]: [8] [10] [12] [9] [11] [13]```

Concatenate the two arrays along the second (horizontal) dimension.

```d3 = cat(2,d1,d2) ```
```d3 = java.lang.Double[][]: [2] [4] [6] [8] [10] [12] [3] [5] [7] [9] [11] [13]```

### Vector Concatenation

This example show the difference between row and column concatenation for vectors. Create two vectors `J1` and `J2`.

```import java.lang.Integer J1 = []; for ii = 1:3 J1 = [J1;Integer(ii)]; end J1```
```J1 = java.lang.Integer[]: [1] [2] [3]```
```J2 = []; for ii = 4:6 J2 = [J2;Integer(ii)]; end J2```
```J2 = java.lang.Integer[]: [4] [5] [6]```

Concatenate by column. Horizontally concatenating two Java vectors creates a longer vector, which prints as a column.

`Jh = [J1,J2]`
```Jh = java.lang.Integer[]: [1] [2] [3] [4] [5] [6]```

Concatenate by row. Vertically concatenating two Java vectors creates a 2-D Java array.

`Jv = [J1;J2]`
```Jv = java.lang.Integer[][]: [1] [2] [3] [4] [5] [6]```

### Note

Unlike MATLAB, a 3x1 Java array is not the same as a Java vector of length 3. Create a 3x1 array.

```import java.lang.Integer arr1 = javaArray('java.lang.Integer',3,1)```
```arr1 = java.lang.Integer[][]: [] [] []```

Create a vector of length 3.

`arr2 = javaArray('java.lang.Integer',3)`
```arr2 = java.lang.Integer[]: [] [] []```