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Logic and integer arithmetic

A look at the bytecodes of the Java virtual machine that perform logical and arithmetic operations

By Bill Venners, JavaWorld.com, 11/11/96

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Overflow in integer operations does not throw any exception in the JVM. The result is merely truncated to fit into the result type. For example, adding shorts 0x7fff and 1 yields 0x8000. This means that the JVM will report that 32,767 + 1 = -32,768, so long as the values being added are shorts and not ints or longs. It is up to the Java programmer to make sure that the appropriate type -- int or long -- is chosen for integer arithmetic in each situation. (If long isn't long enough, the Java programmer should invent a class that implements really long integers and the operations upon them.) Integer division by zero does throw an ArithmeticException, so the programmer should keep in mind that this exception could be thrown and catch it if necessary.



Exposed int: A Java int reveals its inner nature

The following applet lets you play around with the two's-complement format of integers in the JVM. The Max and Min buttons will give you the maximum and minimum values of the int type. By clicking "Max" followed by "++" you can increment beyond the maximum integer and see what happens. Clicking "Min" followed by "--" lets you decrement beyond the minimum integer. Both of these result in overflow, but no exceptions are thrown by the JVM.



You need a Java-enabled browser to view this applet.


Arithmetic opcodes

Integer addition can be performed on ints and longs. Bytes, shorts, and chars are automatically converted to int before they take part in an addition. The following table shows the opcodes that pop the top two values on the stack, add them, and push the result. The type of the values is indicated by the opcode itself, and the result always has the same type as the numbers being added. No exceptions are thrown for any of these opcodes. Overflow is just ignored.



Opcode Operand(s) Description


iadd (none) pops two ints, adds them, and pushes the int result


ladd (none) pops two longs, adds them, and pushes the long result


The next table shows the exception to the rule that arithmetic opcodes take their operands from the stack. The iinc opcode performs an addition on a local variable of type int. The local variable is indicated by the first byte that follows the iinc instruction in the bytecode stream. The amount to add to the local variable is taken from the second byte following the iinc instruction. The second byte is interpreted as a byte type, an eight-bit signed two's-complement number. The local variable and byte are added, and the result is written back to the local variable. This opcode can be used to change a local variable value by any number between and including -128 through 127. This opcode makes for more efficient incrementing and decrementing of variables that are used to control execution of loops, such as for or while. No exceptions are thrown.



Opcode Operand(s) Description


iinc vindex, const adds const to an int at local variable position vindex


Integer subtraction is performed on ints and longs via the following opcodes. Each opcode causes the top two values of the appropriate type to be popped off the stack. The topmost value is subtracted from the value just beneath the topmost value. The result is pushed back onto the stack. No exceptions are thrown by these opcodes.

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Resources
  • Previous Under The Hood articles:
  • The lean, mean virtual machine -- Gives an introduction to the Java virtual machine. Look here to see how the garbage collected heap fits in with the other parts of the JVM.
  • The Java class file lifestyle -- Gives an overview to the Java class file, the file format into which all Java programs are compiled.
  • Java's garbage-collected heap -- Gives an overview of garbage collection in general and the garbage-collected heap of the Java virtual machine in particular.
  • Bytecode basics -- Introduces the bytecodes of the JVM, and discusses primitive types, conversion operations, and stack operations in particular.
  • Floating point arithmetic -- Discusses the Java virtual machine's floating point support and the bytecodes that perform floating point arithmetic.