Double type assignment problem

Double type assignment problem
Double d=1;
Screen display: d=1.0

Double D=1/2;
Screen display: d=0.0

Double D=1.0/2;
Screen display: d=0.5

Double d=1/2.0;
Screen display: d=0.5

Do not know the reason for the study ....
Find some articles that say, multiply or divide between two numbers if you want to keep the accuracy of the results unchanged. Be sure to use the same precision when assigning values to variables. For example:

Double a=1,c=2;
Double d=a/c;
Screen display: d=0.5
It is also possible to define two numbers of which one is a double, and Java converts another int type to a double type at the time of operation. Reference article: Java learning is easy to ignore the small problem