question: private static void walkInfix ( int precedence , ExpressionTree expression , List < ExpressionTree > operands , List < String > operators ) { if ( expression instanceof BinaryTree ) { BinaryTree binaryTree = ( BinaryTree ) expression ; if ( precedence ( binaryTree ) == precedence ) { walkInfix ( precedence , binaryTree . getLeftOperand ( ) , operands , operators ) ; operators . add ( operatorName ( expression ) ) ; walkInfix ( precedence , binaryTree . getRightOperand ( ) , operands , operators ) ; } else { operands . add ( expression ) ; } } else { operands . add ( expression ) ; } }
target:  Accumulate the operands and operators. 