StringBuffer StringBuilder append

StringBuilder is not thread safe. So, it performs better in situations where thread safety is not required.

StringBuffer is implemented by using synchronized keyword on all methods.

StringBuffer

/*      */   public synchronized StringBuffer append(String string)
/*      */   {
/*  197 */     if (string == null) string = String.valueOf(string);
/*  198 */     int adding = string.length();
/*  199 */     int newSize = this.count + adding;
/*  200 */     if (newSize > this.value.length) {
/*  201 */       ensureCapacityImpl(newSize);
/*      */     }
/*  203 */     string.getChars(0, adding, this.value, this.count);
/*  204 */     this.count = newSize;
/*  205 */     return this;
/*      */   }

StringBuilder

/*      */   public StringBuilder append(String string)
/*      */   {
/*  201 */     if (string == null) string = String.valueOf(string);
/*  202 */     int adding = string.length();
/*  203 */     int newSize = this.count + adding;
/*  204 */     if (newSize > this.value.length) {
/*  205 */       ensureCapacityImpl(newSize);
/*      */     }
/*  207 */     string.getChars(0, adding, this.value, this.count);
/*  208 */     this.count = newSize;
/*  209 */     return this;
/*      */   }