关于java 非法的表达？？及代码补充

public String toString()
/*
Pre-condition: none
Post-condition: a String object is returned consisting of the
String representation of all items in the Queue,
from first to last in order, separated by " " and
contained within "<" and ">"
Informally: produce a String representation of the Queue
*/
{
Node c;
String s="";

trace("toString: toString starts");

if (isEmpty())
{
trace("toString: toString ends");
return "<>";
}
else
{
c=first;
while (c != null)
{
s=s.concat(c.getData().toString() + " ");
c=c.getNext();
}
trace("toString: toString ends");

return s;
}
}

public void trace(String s)
/*
Provide trace o

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