CS 208 s21 — Java vs C

1 Video

Here is a video lecture for the material outlined below. It contains sections on

integers, floats, doubles, pointers have the same representation as C
- pointers are called references in Java, and are much more constrained
  - no pointer arithmetic, casting must obey subtyping rules, etc.
- Java specification fixes the sizes of all types
  - e.g., long is 64 bits regardless of machine
Object-oriented language
- can define new classes, use inheritence, Object versions of primitive types (e.g., double and Double)
Another layer between the language and the system as compared to C
- Java compiles to machine code for the Java Virtual Machine (JVM)
- The JVM fetches, interprets, and executes these instructions much like the CPU
- The JVM is implemented in C++, so it's generating the x86-64 instructions to have the CPU execute the Java code

Java initializes elements
Java arrays keep length in an immutable field (array.length)
- immutable means the value cannot be changed (i.e., arrays have a fixed length)
every access triggers a bounds check
- exception if access is out-of-bounds (a Java exception, not an OS exception)
will checking the array length as part of a loop be a big performance hit?
- Length field is likely in cache
- Compiler may store length field in register for loops
- Compiler may prove that some checks are redundant
  - e.g., if the compiler can determine the loop variable is always within the array bound, it doesn't need to check those array accesses

Java uses two-byte Unicode instead of ASCII
- can represent most of the world's alphabets
no null terminator
String objects are read-only
As befits an object-oriented language, a lot of string functionality is provided via String class methods rather than functions that take a string (as in the C library)

virtual method table (vtable) provides pointers to class methods
- each instance stores a pointer to its vtable