Apex is Pass By Value (exec anon)

/**
 * Everything is pass by value in Apex. 
 * When a method is called the runtime pushes a stackframe on the stack and params 
 * are created as local vars, including ref vars. 
 * The object that the ref var points to lives on the heap and can be modified if not immutable. 
 * The param itself is the ref var and is by value like all params in Apex.
 */
public class A {
    public String val { get; set; }
}

void aMethod(A refVar) {
    refVar = null;
    System.debug('var in meth =' + refVar);
}

A refVar = new A();
refVar.val = 'a';
System.debug('before call = ' + refVar);
aMethod(refVar);
System.debug(refVar);

//Output
// 'before call = a'
// 'var in meth = null'
// 'a' -- sill 'a', because the modification in aMethod doesn't stick, because the parameter is pass by value.

//------------------------------------------------------------------------------------
// Here's one that is a bit more explicit. It shows the param getting changed to 
// another object in the method and it still does not persist.
//------------------------------------------------------------------------------------
public class A {
    public String val { get; set; }
}

void aMethod(A refVar) {
    refVar = new A();
    refVar.val = 'b';
    System.debug('var in meth = ' + refVar);
}

A refVar = new A();
refVar.val = 'a';
System.debug('before call = ' + refVar);
aMethod(refVar);
System.debug(refVar);

//Output
// 'before call = a'
// 'var in meth = b'
// 'a' -- sill 'a', because the modification in aMethod doesn't stick, because the parameter is pass by value.