This behavior could be due to a compiler that first checks for matching { and } symbols before looking for other syntax errors. a compiler uses a stack to check for matched symbols such as ( ) , { }, and [ ]. as it reads the java source code, it will only consider opening symbols: ( { [, and closing symbols: ) } ]. if an opening symbol is found in the input file, it is pushed onto a stack. when a closing symbol is read, it is compared to the opener on the top of the stack. if the symbols match, the stack gets popped. if they do not match, the compiler reports an error to the programmer. now imagine processing these tokens, which represent only the openers and closers in a short java program: {{([])}}. as the first four symbols are read — all openers — they get pushed onto the stack.