1.key和value都是list里面对象得属性

public Map getIdNameMap(List accounts) {
    return accounts.stream().collect(Collectors.toMap(Account::getId, Account::getUsername));
}

2.key为属性value为对象本身

public Map getIdAccountMap(List accounts) {
    return accounts.stream().collect(Collectors.toMap(Account::getId, account -> account));
}