CSAPP / CSE 351 学习笔记（Java vs. C）

Data in Java

Yes, Java has pointers - they are called references - however, Java references are much more constrained than C’s general pointers

• Pointers in C can point to any memory address
• References in Java can only point to an object
  • And only to its first element - not to the middle of it

Strings are bounded by hidden length field at beginning of string.

Java Array:

So, it can triggers a bounds-check.

In Java, all variables are references to objects.

Casting

• In C, we can cast any pointer into any other pointer.
• In Java, we can only cast compatible object references.

JVM Operand Stack Example:

iload 1  // push 1st argument from table onto stack
iload 2  // push 2nd argument from table onto stack
iadd     // pop top 2 elements from stack, add together, and
         // push result back onto stack
istore 3 // pop result and put it into third slot in table

• i stands for integer
• a stands for reference
• b stands for byte
• c stands for char
• d stands for double

We have no knowledge of registers or memory locations (each instruction is 1 byte - bytecode)

We leave many information (address) to the C program that interprets bytecode.

Class File Format

• Magic number
• Version of class file format
• Constant pool
• Access flags
• This class
• Super class
• Interfaces
• Fields
• Methods
• Attributes

Other languages for JVMs

JVMs run on so many computes that compilers have been built to translate many other languages to Java bytecode:

(just using Java bytecode)

• AspectJ
• JRuby
• Jython
• Scale
• And many others, even including C

Microsoft’s C# and .NET Framework

• C# has similar motivations as Java
• Virtual machine is called the Common Language Runtime; Common Intermediate Language is the bytecode for C# and other languages in the .NET framework (VB.NET, J#).