Objects and arrays

A look at the bytecodes that deal with objects and arrays in the Java virtual machine

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   0 iconst_5             // Push constant int 5.
   1 iconst_4             // Push constant int 4.
   2 iconst_3             // Push constant int 3.
                          // Create a new multi-dimensional array using constant pool
                          // entry #2 as the class (which is [[[I, an 3D array of ints)
                          // with a dimension of 3.
   3 multianewarray #2 dim #3 <Class [[[I>
   7 astore_0             // Pop object ref into local variable 0: int threeD[][][] = new int[5][4][3];
   8 iconst_0             // Push constant int 0.
   9 istore_1             // Pop int into local variable 1: int i = 0;
  10 goto 54              // Go to section of code that tests outer loop.
  13 iconst_0             // Push constant int 0.
  14 istore_2             // Pop int into local variable 2: int j = 0;
  15 goto 46              // Go to section of code that tests middle loop.
  18 iconst_0             // Push constant int 0.
  19 istore_3             // Pop int into local variable 3: int k = 0;
  20 goto 38              // Go to section of code that tests inner loop.
  23 aload_0              // Push object ref from local variable 0.
  24 iload_1              // Push int from local variable 1 (i).
  25 aaload               // Pop index and arrayref, push object ref at arrayref[index] (gets threeD[i]).
  26 iload_2              // Push int from local variable 2 (j).
  27 aaload               // Pop index and arrayref, push object ref at arrayref[index] (gets threeD[i][j]).
  28 iload_3              // Push int from local variable 3 (k).
                          // Now calculate the int that will be assigned to threeD[i][j][k]
  29 iload_1              // Push int from local variable 1 (i).
  30 iload_2              // Push int from local variable 2 (j).
  31 iadd                 // Pop two ints, add them, push int result (i + j).
  32 iload_3              // Push int from local variable 3 (k).
  33 iadd                 // Pop two ints, add them, push int result (i + j + k).
  34 iastore              // Pop value, index, and arrayref; assign arrayref[index] = value: threeD[i][j][k] = i + j + k;
  35 iinc 3 1             // Increment by 1 the int in local variable 3: ++k;
  38 iload_3              // Push int from local variable 3 (k).
  39 iconst_3             // Push constant int 3.
  40 if_icmplt 23         // Pop right and left ints, jump if left < right: for (...; k < 3;...)
  43 iinc 2 1             // Increment by 1 the int in local variable 2: ++j;
  46 iload_2              // Push int from local variable 2 (j).
  47 iconst_4             // Push constant int 4.
  48 if_icmplt 18         // Pop right and left ints, jump if left < right: for (...; j < 4;...)
  51 iinc 1 1             // Increment by 1 the int in local variable 1: ++i;
  54 iload_1              // Push int from local variable 1 (i).
  55 iconst_5             // Push constant int 5.
  56 if_icmplt 13         // Pop right and left ints, jump if left < right: for (...; i < 5;...)
  59 return

The initAnArray() method merely allocates and initializes a three-dimensional array. This simulation demonstrates how the Java virtual machine handles multidimensional arrays. In response to the multianewarray instruction, which in this example requests the allocation of a three-dimensional array, the JVM creates a tree of one-dimensional arrays. The reference returned by the multianewarray instruction refers to the base one-dimensional array in the tree. In the initAnArray() method, the base array has five components -- threeD[0] through threeD[4]. Each component of the base array is itself a reference to a one-dimensional array of four components, accessed by threeD[0][0] through threeD[4][3]. The components of these five arrays are also references to arrays, each of which has three components. These components are ints, the elements of this multidimensional array, and they are accessed by threeD[0][0][0] through threeD[4][3][2].

In response to the multianewarray instruction in the initAnArray() method, the Java virtual machine creates one five-dimensional array of arrays, five four-dimensional arrays of arrays, and twenty three-dimensional arrays of ints. The JVM allocates these 26 arrays on the heap, initializes their components such that they form a tree, and returns the reference to the base array.

To assign an int value to an element of the three-dimensional array, the JVM uses aaload to get a component of the base array. Then the JVM uses aaload again on this component -- which is itself an array of arrays -- to get a component of the branch array. This component is a reference to a leaf array of ints. Finally the JVM uses iastore to assign an int value to the element of the leaf array. The JVM uses multiple one-dimensional array accesses to accomplish operations on multidimensional arrays.

To drive the simulation, just press the Step button. Each press of this button will cause the Java virtual machine to execute one bytecode instruction. To start the simulation over, press the Reset button. To cause the JVM to repeatedly execute bytecodes with no further coaxing on your part, press the Run button. The JVM will then execute the bytecodes until the Stop button is pressed. The return instruction in the bytecode sequence generated by javac has been replaced by a breakpoint instruction in the simulation's bytecode sequence. In this case, the breakpoint instruction just causes the simulator to stop. The text area at the bottom of the applet describes the next instruction to be executed. Happy clicking.

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Click here for the source code of three-dimensional array.

Bill Venners has been writing software professionally for 12 years. Based in Silicon Valley, he provides software consulting and training services under the name Artima Software Company. Over the years he has developed software for the consumer electronics, education, semiconductor, and life insurance industries. He has programmed in many languages on many platforms: assembly language on various microprocessors, C on Unix, C++ on Windows, Java on the Web. He is author of the book: Inside the Java Virtual Machine, published by McGraw-Hill.

Learn more about this topic

  • 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 Java Virtual Machine.
  • 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 Java Virtual Machine, and discusses primitive types, conversion operations, and stack operations in particular.
  • Floating Point Arithmetic -- Describes the Java Virtual Machine's floating point support and the bytecodes that perform floating point operations.
  • Logic and Integer Arithmetic -- Describes the Java Virtual Machine's support for logical and integer arithmetic, and the relevant bytecode instructions.

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