STRING BUFFERS

1. INTRODUCTION

String Buffers are immutable. For example a call to String class method toLowerCase() will not alter thecstring that is passed to it but create a new string. We van change individual elements of a string using replace method. We can also, of course assign an entirely new value to a String instance. But what if we wish to vary the size of an existing string by adding or subtracting characters? To do this we need to crearte an instance of the class StringBuffer. A string buffer is like a string except that it is mutable, i.e. we can add to the string.

There are three StringBuffer constructors avaialbale:

STRING BUFFER CONSTRUCTORS
`StringBuffer()`	Constructs a string buffer with no characters in it and an initial capacity of 16 characters.
`StringBuffer(int length)`	Constructs a string buffer with no characters in it and an initial capacity specified by the length argument.
`StringBuffer(String str)`	Constructs a string buffer so that it represents the same sequence of characters as the string argument.

Once an instance of the class StringBuffer has been created we can allow the object to expand or retract as required using the methods supplied in the StringBuffer class. There are two principal types of method to do this:

Append methods adds the string representation of its argument to the StringBuffer object.
Insert methods insert the string representation of its argument into the StringBuffer object at a given offset.

2. EXAMPLE

Consider the fourty character buffers that presented in Table 1. Here is sample illustrate the Insert stringsBuffer.

; Sample of how to Insert StringsBuffer
;
; -------------------------------------------
             ; DOS CALL Constants
             BIOS_CALL EQU 21h
             STRING_OUT_FN EQU 09h
             CHAR_IN_FN EQU 01h
             CHAR_OUT_FN EQU 02h
             EXIT_TO_DOS_FN EQU 4Ch
; -------------------------------------------
; Assembler directives
  .model small
; -------------------------------------------
; S T A C K
             .stack 100h
; -------------------------------------------
; D A T A 
             .data
BUFF1 DB 40 dup('              '), '$'
             BUFF2 DB 40 dup(' '), '$'
             TEMPSTR DB 40 dup(' '), '$'
BUFFER1_MSG DB              0AH,0Ah,0Dh,"Buffer_1 = $"
             BUFFER2_MSG DB 0Ah,0Dh,"Buffer_2 = $"
OPENING_MSG DB              0Ah,0Dh,"PROGRAM 4: STRING BUFFER"
             DB 0Ah,0Dh,"------------------------$"
             PROMPT_MSG DB 0Ah,0Dh,"Choose operation (1=Buffer1, 2=Buffer2,              Q=quit): $"
             STR_PROMPT_MSG DB 0Ah,0Dh,"String input: $"
             POS_PROMPT_MSG DB 0Ah,0Dh,"Position to insert: $"
             GOODBYE_MSG DB 0Ah,0Dh,"Program Ending",0Ah,0Dh,'$'
TheChar DB ?              ;holds the user's key press.
             ThePos DB ?
; -------------------------------------------
; C O D E 
             .code
; -------------------------------------------
             ; M A I N
             ; The entire program is here!
             MAIN PROC
; Initialize              Data Segment Register
mov AX,@DATA
             mov DS,AX
             mov ES,AX
             ;
             ; Display Title
             ;
             mov AH,STRING_OUT_FN
             lea DX,OPENING_MSG
             int BIOS_CALL 
; -------------------------------------------
             ; M A I N L O O P
             ; (not really a loop but it deserves a lot of attention)
             ;
             MAINLOOP:
             ;
             ; Display Contents of BUFF1
             ;
             PUSH DX
             mov AH,STRING_OUT_FN
             lea DX,BUFFER1_MSG
             int BIOS_CALL
LEA SI,BUFF1
             MOV BX,40
             CALL DISP_STR
             ; Display Contents of BUFF2
             
             mov AH,STRING_OUT_FN
             lea DX,BUFFER2_MSG
             int BIOS_CALL
LEA SI,BUFF2
             MOV BX,40
             CALL DISP_STR             
             ;
             ; Prompt User to choose an operation
             ;
             mov AH,STRING_OUT_FN
             lea DX,PROMPT_MSG
             int BIOS_CALL
POP DX
;
             ; Get the user's selection
             ;
             mov AH, CHAR_IN_FN
             int BIOS_CALL
             mov TheChar,AL 
;
             ; Reacts accordingly to the selection

 ; if the char              is q
             cmp TheChar,'Q'
             je GOODBYE
             cmp TheChar,'q'
             je GOODBYE             
             ; else input the string
             ;
             ; Output reply message
             ;
             PUSH DX
mov AH,STRING_OUT_FN
             lea DX,STR_PROMPT_MSG
             int BIOS_CALL
; Read a string
             LEA DI,TEMPSTR
             CALL READ_STR ; CALLS READ STRING          
             POP DX
             PUSH BX ; Preserves the value in BX
;
             ; Prompt User to choose an position
             ;
             PUSH DX
mov AH,STRING_OUT_FN
             lea DX,POS_PROMPT_MSG
             int BIOS_CALL
POP DX             
             ;
             ; Get the position
             ;
             mov AH, CHAR_IN_FN
             int BIOS_CALL
             mov ThePos,AL          
             ;
             ; Output User's character
             ;
             ; mov AH,CHAR_OUT_FN
             ; mov DL,TheChar
             ; int BIOS_CALL

 POP AX
             CMP TheChar,'2'
             JE BUFF2LEN
             ADD DH,AL

BUFF2LEN:
             ADD DL,AL ; Calculates the length of buffer2

 CALL INDEC ;              Turns the position character into
             ; its decimal value
CALL INSERT_STR              ; CALLS INSERT STRING
JMP MAINLOOP
             ;
             ; Output Goodbye message
             ;
             GOODBYE:
             mov AH,STRING_OUT_FN
             lea DX,GOODBYE_MSG
             int BIOS_CALL
;
             ; Exit to DOS
             ;
             mov AH,EXIT_TO_DOS_FN
             int BIOS_CALL
             MAIN ENDP
; ----------------------------------------------------
             ; 
             ; P R O C E D U R E 
             ;
             ; S E C T I O N
             ;
             ; ----------------------------------------------------

; ----------------------------------------------------
             ; Procedure: R E A D _ S T R
             ;
             ; input: a string from the user
             ;
             ; output: ???stores the string in a variable called
             STRING
             ;
             READ_STR PROC
PUSH AX
             PUSH DI
CLD
             XOR BX,BX
             MOV AH,1
             INT 21h

WHILE1:
             CMP AL,0DH
             JE END_WHILE1

; if char is              backspace
             CMP AL,8H
             JNE ELSE1             
             ; then
             DEC DI
             DEC BX
             JMP READ
             
             ELSE1:
             STOSB
             INC BX
             ;CMP BX,8
             ;JLE OKAY_TO_CONTINUE
             ;PUSH DX

 ;MOV DL,07h              ; Sound alarm
             ;MOV AH,CHAR_OUT_FN ; (IS NOT WORKING)
             ;INT 21h ;             
             ;POP DX
;JMP END_WHILE
OKAY_TO_CONTINUE:             
             READ:
             INT 21H
             JMP WHILE1             
             END_WHILE1:
             POP DI
             POP AX             
             RET
             READ_STR ENDP
; ----------------------------------------------------
             ; Procedure: I N S E R T _ S T R
             ;
             ; Inserts string1 into string2 at a specified position
             ;
             ; Input: SI = offset of string1 [The string input]
             ; DI = offset of string2 [Pointer to the buffer, either #1 or #2]
             ; AX = length of string1
             ; DX = length of string2
             ; BX = position to insert string2
             ;
             ; Output: None
             INSERT_STR PROC             
             PUSH AX
             PUSH BX
             PUSH CX
             PUSH DX
CMP TheChar,'1'
             JE BUFFER1
             CMP TheChar,'2'
             JE BUFFER2
JMP EXIT1
BUFFER1:
             LEA DI,[BUFF1 + BX]
             ;JMP CONTINUE1
PUSH AX
             ADD AX,BX
             LEA DI,[BUFF1 + AX]
             POP AX
JMP CONTINUE1
BUFFER2:
             LEA DI,[BUFF2 + BX]
             JMP CONTINUE2
CONTINUE1:
             LEA SI,[BUFF1 + BX]
             JMP CONTINUE3
CONTINUE2:
             LEA SI,[BUFF2 + BX]
             JMP CONTINUE3
CONTINUE3:
             ;LEA SI,TEMPSTR
CLD
             CMP DX,0
             JNE CONTINUE4
             MOV CX,DX
             REP MOVSB
CONTINUE4:
             LEA SI,TEMPSTR
CLD
             MOV CX,AX
             REP MOVSB
EXIT1: 
             POP DX
             POP CX
             POP BX
             POP AX 
             RET
             INSERT_STR ENDP
; ----------------------------------------------------
             ; Procedure: D I S P _ S T R
             ;
             ; Reads a number in range -32768 to 32767
             ;
             ; Input: none
             ;
             ; Output: BX: binary equivalent of number
             DISP_STR PROC
             PUSH AX
             PUSH BX
             PUSH CX
             PUSH DX
             PUSH SI
MOV CX,BX
             JCXZ P_EXIT
             CLD
             MOV AH,2
TOP:
             LODSB
             MOV DL,AL
             INT 21H
             LOOP TOP
P_EXIT:
             POP SI
             POP DX
             POP CX
             POP BX
             POP AX          
             RET
             DISP_STR ENDP
; ----------------------------------------------------
             ; Procedure: I N D E C
             ; Input: none
             ; Output: BX: binary equivalent of number
             INDEC PROC
             PUSH AX
             PUSH CX
             PUSH DX
; convert character to a digit
             AND BX,BX
             MOV BL,ThePos
             AND BX,000FH
POP DX
             POP CX
             POP AX
             RET
             INDEC ENDP
             END MAIN

Table 1:String buffer illustration

String buffers are also usefull where we wish to pass strings to methods where we wish to change the values of the string object. We hope this sample help you to understand how to use insert stringBuffers.

Created and maintained by Fanh Khcone. Last updated 01 September 2000

STRING BUFFERS

CONTENTS

1. INTRODUCTION

2. EXAMPLE