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Bytecode basics

A first look at the bytecodes of the Java virtual machine

By Bill Venners, JavaWorld.com, 09/01/96

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Primitive types

The JVM supports seven primitive data types. Java programmers can declare and use variables of these data types, and Java bytecodes operate upon these data types. The seven primitive types are listed in the following table:

Type Definition
byte one-byte signed two's complement integer
short two-byte signed two's complement integer
int 4-byte signed two's complement integer
long 8-byte signed two's complement integer
float 4-byte IEEE 754 single-precision float
double 8-byte IEEE 754 double-precision float
char 2-byte unsigned Unicode character

The primitive types appear as operands in bytecode streams. All primitive types that occupy more than 1 byte are stored in big-endian order in the bytecode stream, which means higher-order bytes precede lower-order bytes. For example, to push the constant value 256 (hex 0100) onto the stack, you would use the sipush opcode followed by a short operand. The short appears in the bytecode stream, shown below, as "01 00" because the JVM is big-endian. If the JVM were little-endian, the short would appear as "00 01". 

 // Bytecode stream: 17 01 00
// Dissassembly:
sipush 256;      // 17 01 00


Java opcodes generally indicate the type of their operands. This allows operands to just be themselves, with no need to identify their type to the JVM. For example, instead of having one opcode that pushes a local variable onto the stack, the JVM has several. Opcodes iload, lload, fload, and dload push local variables of type int, long, float, and double, respectively, onto the stack.

Pushing constants onto the stack

Many opcodes push constants onto the stack. Opcodes indicate the constant value to push in three different ways. The constant value is either implicit in the opcode itself, follows the opcode in the bytecode stream as an operand, or is taken from the constant pool.

Some opcodes by themselves indicate a type and constant value to push. For example, the iconst_1 opcode tells the JVM to push integer value one. Such bytecodes are defined for some commonly pushed numbers of various types. These instructions occupy only 1 byte in the bytecode stream. They increase the efficiency of bytecode execution and reduce the size of bytecode streams. The opcodes that push ints and floats are shown in the following table:

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