What am I doing wrong in creating an array with even numbers?

The increment you use only to compensate for the fact that arrays start with index 0. What you are missing is a multiplication by 2 to use only even numbers.

public class Matriz2 {

    public static void main(String[] args) {
        // int matriz1[][] = new int[5][5]; // no recomiendo declarar un arreglo así,
        // aun que esta permitido, es menos claro
        // el formato generalmente preferido "Tipo[] var = new Tipo[size];
        int[][] matriz1 = new int[10][5]; // si quieres llegar a 10, necesitas 10 filas
        int i = 0;
        int j = 0;
        while (i < matriz1.length) {
            //matriz1[i][j]= i*matriz1.length;
            //System.out.print(matriz1[i][j]+" ");
            j =0;
            while (j < matriz1[i].length) {
                matriz1[i][j] = i*10 + (j+1)*2; 
                System.out.print(String.format("%02d ", matriz1[i][j]));
                j++; // tienes que incrementar dentro del loop
            }
            System.out.println();
            i++; // idem
        }
    }

}

Result:

02 04 06 08 10 
12 14 16 18 20 
22 24 26 28 30 
32 34 36 38 40 
42 44 46 48 50 
52 54 56 58 60 
62 64 66 68 70 
72 74 76 78 80 
82 84 86 88 90 
92 94 96 98 100 

For examples of how to declare arrays, see here .

added as per comment

for odd ones you only have to change the calculation of the second dimension:

           matriz1[i][j] = i*10 + j*2 +1;

with a ternary, you could declare a boolean before and then differentiate in the same code:

           boolean par;

           matriz1[i][j] = i*10 + (par) ? (j+1)*2 : j*2 +1;

Seeing that it is a sequence of numbers (2,4,6,8,...100), we need a variable kso that it increases in each iteration in both cycles.

I leave you your own code, resolved and commented.

import java.lang.Math; // headers MUST be above the first class

// one class needs to have a main() method
public class HelloWorld
{
  public static void main(String[] args) {
    int matriz1[][] = new int[10][5];//se necesita 10 filas
    int i = 0;
    int j = 0;
    int k=2;//debe iniciar en 2(ver imagen)
    while (i < matriz1.length) {
        matriz1[i][j]= k;
        k=k+2;//se incrementa en ambos ciclos
        System.out.print(matriz1[i][j]+" ");
        while (j < matriz1[i].length-1) {//j llegara a ser mayor o igual que matriz1[i].length-1, por lo tanto j debe iniciar en 0 nuevamente(ver abajo) 
          matriz1[i][j] = k;
          System.out.print(matriz1[i][j]+" ");
          k=k+2; //se incrementa en ambos ciclos
          j++;
        }

      i++;
      j=0;//se necesita iniciar en por cada iteracion de i 
      System.out.println();
    }

}
}

I would recommend that you use forwhen you already know when it will finish, in your case it is known from the beginning that it ishould iterate 10 times and it jshould iterate 5 times, and on the contrary, when you do not know when it will finish, you should use it whilecheck my answer to this topic , for vs while - LOOP solving with for:

public class HelloWorld
{
  public static void main(String[] args) {
    int matriz1[][] = new int[10][5];

    int k=2;
    for (i=0;i < matriz1.length;i++) {
        matriz1[i][j]= k;
        k=k+2;
        System.out.print(matriz1[i][j]+" ");
        for (j=0;j < matriz1[i].length-1;j++) {
          matriz1[i][j] = k;
          System.out.print(matriz1[i][j]+" ");
          k=k+2;
        }
      System.out.println();
    }

}
}

Hi, I'll give you a solution to generate arrays of pairs with the generalized array size.

I have divided the functionality into 2 methods so that it is well defined what each part of the code does.

public class PairMatrixGenerator {

    private static int [][] matrix;

    public static void main(String []args){
        generateMatrix(10,5);
        printMatrix();
    }

    public static void generateMatrix(int rows,int columns){
        matrix = new int[rows][columns];
        int i = 0;
        int j = 0;
        while (i < rows){
            while(j < columns){
                matrix[i][j] = 2 * (i * columns + j + 1);
                j++;
            }
            j=0;
            i++;
        }
    }

    public static void printMatrix(){
        int i = 0;
        int j = 0;
        while (i < matrix.length){
            while(j < matrix[i].length){
                System.out.printf("%02d ",matrix[i][j]);
                j++;
            }
            System.out.println();
            j=0;
            i++;
        }
    }
}

If you want to put everything in the main it would be something like this

public class PairMatrixGenerator {

    public static void main(String []args){
        int [][] matrix = new int[10][5];
        int i = 0;
        int j = 0;
        while (i < matrix.length){
            while(j < matrix[i].length){
                matrix[i][j] = 2 * (i * columns + j + 1);
                System.out.printf("%02d ",matrix[i][j]);
                j++;
            }
            System.out.println();
            j=0;
            i++;
        }
    }        
}