【2014-11-23】《The Hardware/Software Interface》– Section 11

1. Data in Java
• Java has pointers – they are called ‘referenes’
• Null is typically represented as 0
• Characters and strings
  • Two-byte Unicode instead of ASCII
    • Represents most of the world’s alphabets
  • String not bounded by a ‘\0’(null character)
    • Bounded by hidden length field at beginning of string
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• Arrays
  • Every element initialized to 0
  • Bounds specified in hidden fields at start of array(int – 4 bytes)
    • array.length return value of this field
  • Every access trigger a bounds-check
    • code is added to ensure the index is within bounds
    • Exception if out-of-bounds
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• Data structures (Object) in Java
  • Objects can only include primitive data types
    • include complex data types(arrays, other objects, ec.) using references
• Pointers/References
• 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
• In Java, all variables are reference to objects
• use r.a notation
• Casting in C
• we can cast any pointer into any other pointer
• Casting in Java
• can only cast compatible object references
• Java Methods
• Methods in Java are just functions but with an extra argument: a reference to the object whose method is being called
• Vitual machine and runtime environment
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