garbage collection - Does chaining methods vs making temporary variables in Java impact memory allocation? -


in java, if i'm performing multiple methods on object, can chain them, or can make temporary variable, so

chaining

    system.out.println( str.substring(0,4).substring(0,2));

temp variable

    string tmp = str.substring(0,4);     tmp = tmp.substring(0,2);     system.out.println(tmp);

obviously, difference negligible in example, make impact when you're doing on thousands of strings/some other object.

my question is, 1 of these more "efficient" in terms of not making object allocations or filling heap (and making gc called sooner)?

i tried compare bytecodes of 2 in loop on couple strings, looks similar, sans last few lines. don't understand bytecode calls, i'm not sure if of these have allocating new objects.

compiled "testnotmp.java" public class testnotmp {   public testnotmp();     code:        0: aload_0        1: invokespecial #1                  // method java/lang/object."<init>":()v        4: return    public static void main(java.lang.string[]);     code:        0: iconst_4        1: anewarray     #2                  // class java/lang/string        4: dup        5: iconst_0        6: ldc           #3                  // string these        8: aastore        9: dup       10: iconst_1       11: ldc           #4                  // string are__       13: aastore       14: dup       15: iconst_2       16: ldc           #5                  // string some_       18: aastore       19: dup       20: iconst_3       21: ldc           #6                  // string strings       23: aastore       24: astore_1       25: aload_1       26: astore_2       27: aload_2       28: arraylength       29: istore_3       30: iconst_0       31: istore        4       33: iload         4       35: iload_3       36: if_icmpge     69       39: aload_2       40: iload         4       42: aaload       43: astore        5       45: getstatic     #7                  // field java/lang/system.out:ljava/io/printstream;       48: aload         5       50: iconst_0       51: iconst_4       52: invokevirtual #8                  // method java/lang/string.substring:(ii)ljava/lang/string;       55: iconst_0       56: iconst_2       57: invokevirtual #8                  // method java/lang/string.substring:(ii)ljava/lang/string;       60: invokevirtual #9                  // method java/io/printstream.println:(ljava/lang/string;)v       63: iinc          4, 1       66: goto          33       69: return }  public class testtmp {   public testtmp();     code:        0: aload_0        1: invokespecial #1                  // method java/lang/object."<init>":()v        4: return    public static void main(java.lang.string[]);     code:        0: iconst_4        1: anewarray     #2                  // class java/lang/string        4: dup        5: iconst_0        6: ldc           #3                  // string these        8: aastore        9: dup       10: iconst_1       11: ldc           #4                  // string are__       13: aastore       14: dup       15: iconst_2       16: ldc           #5                  // string some_       18: aastore       19: dup       20: iconst_3       21: ldc           #6                  // string strings       23: aastore       24: astore_1       25: aload_1       26: astore_2       27: aload_2       28: arraylength       29: istore_3       30: iconst_0       31: istore        4       33: iload         4       35: iload_3       36: if_icmpge     77       39: aload_2       40: iload         4       42: aaload       43: astore        5       45: aload         5       47: iconst_0       48: iconst_4       49: invokevirtual #7                  // method java/lang/string.substring:(ii)ljava/lang/string;       52: astore        6       54: aload         6       56: iconst_0       57: iconst_2       58: invokevirtual #7                  // method java/lang/string.substring:(ii)ljava/lang/string;       61: astore        6       63: getstatic     #8                  // field java/lang/system.out:ljava/io/printstream;       66: aload         6       68: invokevirtual #9                  // method java/io/printstream.println:(ljava/lang/string;)v       71: iinc          4, 1       74: goto          33       77: return }

in example, you're working strings, immutable. in code:

str.substring(0,4).substring(0,2)

the first call substring must generate new string object because str cannot modified. similarly, second call substring on new string object create new string object.

the difference in bytecodes result of order in compiler calls methods. in testtmp case, string manipulation occurs before call printstream. testnotmp, string calls happen within printstream call, logical when @ code.

to answer question, make no difference in terms of object allocation , therefore gc impact.


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