First, we should be familiar with some terms used for assignment.

Expression: There are lots of different kind of expressions. The most common one is the arithmetic expression such as 3+2 and it can be evaluated to produce the value 5. We also use expressions to create a new object by instantiating a class."new Elephant()" is an expression.
Value of the expression: It is the reference to an object.
Notice: A variable can also be an expression and its value is the reference assigned to it.

After we declared a variable, we usually assign a value to the variable which means the variable got a reference to an expression. It can be illustrated using a picture:

We can see from the picture that the order of the assignment is from the right side of equation to the left side of the equation.

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Examples of assignment in java code
(The basic assignment of variables is the same for different variables.)

identifier = expression
e.g. elephant= new Elephant();
We can also combined the declaration and the assignment together as
type identifier = expression;
e.g. Elephant elephant= new Elephant();

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Example of exchanging values between variables

This is an example to exchange the value of the variable a and variable b. The picture shows what is going on inside the machine when java executes the codes.