boolean, byte, short, int, long, float, double, char are allowed types and they have 1-bit, 1 byte, 2 bytes, 4 bytes, 8 bytes (two's complement), 4 bytes (IEEE 754), 8 bytes (IEEE 754) and 2 bytes. Only char and boolean types are unsigned in Java. char is not the same as byte, int, short or Strings. 

There is no restriction on the length of variable names. However, you'd better to use understandable and meaningful names for variables. 

Although Java handles the conversion between numeric values, it is better to explicitly warn the compiler to make casting. In my opinion, you should always explicitly define the type of result. Although Java does its own truncation and/or rounding, this may yield in the loss of precision in one of your trials. Especially when the results are too big or too small enough to store in the resulting variable, Java will possibly do it wrong. 

It is possible to convert strings to integers or vice versa easily in Java. toString() and valueOf() methods performs this job. 

int i = Integer.valueOf("22").intValue(); // assigns the number 22 to integer i 
double d = Double.valueOf("3.14").doubleValue(); // assigns the number 3.14 to double d

Since array is an indexed collection of similar type of data, we should first specify its data type. Global array declaration is as follows: 

int[] k = new int[10];

For multidimensional arrays, we use: 

int[][] table = new int[10][15];

int[][] matrix = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12} };